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Objectives: The Sherpa ethnic group living at altitude in Nepal may have experienced natural selection in response to chronic hypoxia. We have previously shown that Sherpa in Kathmandu (1400 m) possess larger spleens and a greater apnea-induced splenic contraction compared to lowland Nepalis. This may be significant for exercise capacity at altitude as the human spleen responds to stress-induced catecholamine secretion by an immediate contraction, which results in transiently elevated hemoglobin concentration ([Hb])., Methods: To investigate splenic contraction in response to exercise at high-altitude (4300 m; P b  = ~450 Torr), we recruited 63 acclimatized Sherpa (29F) and 14 Nepali non-Sherpa (7F). Spleen volume was measured before and after maximal exercise on a cycle ergometer by ultrasonography, along with [Hb] and oxygen saturation (SpO 2 )., Results: Resting spleen volume was larger in the Sherpa compared with Nepali non-Sherpa (237 ± 62 vs. 165 ± 34 mL, p < .001), as was the exercise-induced splenic contraction (Δspleen volume, 91 ± 40 vs. 38 ± 32 mL, p < .001). From rest to exercise, [Hb] increased (1.2 to 1.4 g.dl -1 ), SpO 2 decreased (~9%) and calculated arterial oxygen content (CaO 2 ) remained stable, but there were no significant differences between groups. In Sherpa, both resting spleen volume and the Δspleen volume were modest positive predictors of the change (Δ) in [Hb] and CaO 2 with exercise (p-values from .026 to .037 and R 2 values from 0.059 to 0.067 for the predictor variable)., Conclusions: Larger spleens and greater splenic contraction may be an adaptive characteristic of Nepali Sherpa to increase CaO 2 during exercise at altitude, but the direct link between spleen size/function and hypoxia tolerance remains unclear., (© 2024 The Authors. American Journal of Human Biology published by Wiley Periodicals LLC.)