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Purpose: This multi-study programme investigated the optimal concentration of l-menthol delivered as an oral mouth rinse to modulate thermo-behaviour during exercise in a hot environment (35 °C).In study 1, 38 participants completed a survey to establish an effective and tolerable range of l-menthol concentration. 31 participants completed an RPE-protocol examining 1. the dose–response effect of l-menthol mouth rinse on exercise performance (n = 16) and 2. the temporal effectiveness of administering l-menthol in an incremental and decremental dosing pattern (n = 15). Power output, heart rate, body core temperature and thermal sensation were reported throughout.The optimal menthol concentration for peak power was between 0.01 and 0.1% (~ 6% increase, P < 0.05) and 0.5% (~ 9% increase, P < 0.05) with respect to control. Work completed was increased at 0.01% (~ 5%, P < 0.05), at 0.1% (~ 3%, P < 0.05) and had a detrimental effect at 0.5% (− 10% decrease, P < 0.05). There were no differences between an ascending dose protocol (0.01 to 0.5%), descending dose protocol (0.5–0.01%) or a constant 0.01% dose protocol. There were no reported differences in body core temperature or heart rate across trials (P > 0.05).The optimal dose of l-menthol when delivered via oral rinsing is between 0.01 and 0.1%. At lower concentrations, l-menthol appears to be less effective and at higher concentrations (> 0.5%) l-menthol appears to elicit greater irritation and may not positively modulate thermo-behaviour during exercise in a hot environment.Method: This multi-study programme investigated the optimal concentration of l-menthol delivered as an oral mouth rinse to modulate thermo-behaviour during exercise in a hot environment (35 °C).In study 1, 38 participants completed a survey to establish an effective and tolerable range of l-menthol concentration. 31 participants completed an RPE-protocol examining 1. the dose–response effect of l-menthol mouth rinse on exercise performance (n = 16) and 2. the temporal effectiveness of administering l-menthol in an incremental and decremental dosing pattern (n = 15). Power output, heart rate, body core temperature and thermal sensation were reported throughout.The optimal menthol concentration for peak power was between 0.01 and 0.1% (~ 6% increase, P < 0.05) and 0.5% (~ 9% increase, P < 0.05) with respect to control. Work completed was increased at 0.01% (~ 5%, P < 0.05), at 0.1% (~ 3%, P < 0.05) and had a detrimental effect at 0.5% (− 10% decrease, P < 0.05). There were no differences between an ascending dose protocol (0.01 to 0.5%), descending dose protocol (0.5–0.01%) or a constant 0.01% dose protocol. There were no reported differences in body core temperature or heart rate across trials (P > 0.05).The optimal dose of l-menthol when delivered via oral rinsing is between 0.01 and 0.1%. At lower concentrations, l-menthol appears to be less effective and at higher concentrations (> 0.5%) l-menthol appears to elicit greater irritation and may not positively modulate thermo-behaviour during exercise in a hot environment.Results: This multi-study programme investigated the optimal concentration of l-menthol delivered as an oral mouth rinse to modulate thermo-behaviour during exercise in a hot environment (35 °C).In study 1, 38 participants completed a survey to establish an effective and tolerable range of l-menthol concentration. 31 participants completed an RPE-protocol examining 1. the dose–response effect of l-menthol mouth rinse on exercise performance (n = 16) and 2. the temporal effectiveness of administering l-menthol in an incremental and decremental dosing pattern (n = 15). Power output, heart rate, body core temperature and thermal sensation were reported throughout.The optimal menthol concentration for peak power was between 0.01 and 0.1% (~ 6% increase, P < 0.05) and 0.5% (~ 9% increase, P < 0.05) with respect to control. Work completed was increased at 0.01% (~ 5%, P < 0.05), at 0.1% (~ 3%, P < 0.05) and had a detrimental effect at 0.5% (− 10% decrease, P < 0.05). There were no differences between an ascending dose protocol (0.01 to 0.5%), descending dose protocol (0.5–0.01%) or a constant 0.01% dose protocol. There were no reported differences in body core temperature or heart rate across trials (P > 0.05).The optimal dose of l-menthol when delivered via oral rinsing is between 0.01 and 0.1%. At lower concentrations, l-menthol appears to be less effective and at higher concentrations (> 0.5%) l-menthol appears to elicit greater irritation and may not positively modulate thermo-behaviour during exercise in a hot environment.Conclusion: This multi-study programme investigated the optimal concentration of l-menthol delivered as an oral mouth rinse to modulate thermo-behaviour during exercise in a hot environment (35 °C).In study 1, 38 participants completed a survey to establish an effective and tolerable range of l-menthol concentration. 31 participants completed an RPE-protocol examining 1. the dose–response effect of l-menthol mouth rinse on exercise performance (n = 16) and 2. the temporal effectiveness of administering l-menthol in an incremental and decremental dosing pattern (n = 15). Power output, heart rate, body core temperature and thermal sensation were reported throughout.The optimal menthol concentration for peak power was between 0.01 and 0.1% (~ 6% increase, P < 0.05) and 0.5% (~ 9% increase, P < 0.05) with respect to control. Work completed was increased at 0.01% (~ 5%, P < 0.05), at 0.1% (~ 3%, P < 0.05) and had a detrimental effect at 0.5% (− 10% decrease, P < 0.05). There were no differences between an ascending dose protocol (0.01 to 0.5%), descending dose protocol (0.5–0.01%) or a constant 0.01% dose protocol. There were no reported differences in body core temperature or heart rate across trials (P > 0.05).The optimal dose of l-menthol when delivered via oral rinsing is between 0.01 and 0.1%. At lower concentrations, l-menthol appears to be less effective and at higher concentrations (> 0.5%) l-menthol appears to elicit greater irritation and may not positively modulate thermo-behaviour during exercise in a hot environment. [ABSTRACT FROM AUTHOR]