testing increases memory of a topic studied more than additional study or training. The mechanisms by which this occurs are not clearly understood. Testing can be stressful and studies suggest that the stress hormone cortisol has modulating effects on memory, predominantly in men. The aim of this study was to investigate whether cardiopulmonary resuscitation (CPR) skills testing induce a cortisol increase, whether the cortisol increase enhances retention of CPR skills, and how this relates to gender.we randomised a convenience sample of medical students attending a mandatory course to one intervention and one control group. Students received a 4-hour course on CPR skills. During the final half-hour of the intervention course, participants were tested in CPR scenarios, whereas the control group underwent additional training. We assessed learning outcomes 2 weeks later by rating student performance in a CPR scenario using a checklist and a single blinded assessor. We measured salivary cortisol pre-course, half an hour before the end of the course and post-course, and compared learning outcomes and cortisol responses between groups and genders.in total, 146 of 202 (72%) students completed the study. We found a significant difference in learning outcome between the intervention and control groups for both genders (mean ? standard deviation, 5.0 ? 3.5; p = 0.006). We found a significant effect of increase in cortisol on learning outcome in men. The correlation between learning outcome and cortisol increase was medium to large for men (r = 0.38), but not for women (r = -0.05).cardiopulmonary resuscitation skills testing induces a rise in cortisol in men, which is related to the better retention of skills in men. Cortisol modulates test-enhanced learning in men.