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Background It is recognized that, as a result of variation in tissue anatomy, current auto-injectors may have insufficient needle length to achieve successful intramuscular agent delivery in a number of patients. The Zeneo® auto-injector is a novel prefilled, single-use, needle-free device currently in development for intradermal, subcutaneous, and intramuscular agent delivery across a variety of clinical indications. We aimed to evaluate delivery depth of the device calibrated at pressure appropriate for intramuscular (IM) administration. Methods This was a prospective single-center study in healthy adult volunteers, in whom each received a single injection of saline into the anterolateral thigh. Using sequential MRI scans, we measured skin-to-muscle distance (STMD) agent delivery depth, and the success of IM agent penetration. Device dynamic pressure measurements were also recorded. Results Results are reported for 37 subjects with evaluable MRI scans; 19 men, 18 women; mean age 38 years (range 20–58); mean BMI 27.0 kg/m2 (range 21.2–30.8 kg/m2). Mean STMD values were 18.6 mm (range 13.4–23.6 mm) in women and 10.0 mm (range 5.0–21.7 mm) in men, with gender differences due primarily to greater subcutaneous thickness in women. A trend for greater STMD in subjects with BMI greater than 25 kg/m2 was seen. Mean injectate penetration depths of 30.1 mm (range 20.2–45.6 mm) were observed with values similar in male and female subjects. Successful IM delivery was reported in 95% of subjects. When failure occurred, this was not due to inadequate injection depth. Device pressure (P max) had the greatest influence on injectate muscle penetration. Conclusion Use of the Zeneo® auto-injector achieves delivery depth that ensures intramuscular delivery in both men and women, regardless of BMI. Consistent with other reported data, STMD is greater in women. Funding Crossject.