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 (Pmax) 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.