PURPOSE: The purpose of the study was to investigate the effects of 750 mg of soybean-derived phosphatidylserine, administered daily for 10 d, on exercise capacity, oxygen uptake kinetic response, neuroendocrine function, and feeling states during exhaustive intermittent exercise. METHODS: Following preliminary testing, fourteen active males completed a staged intermittent exercise protocol on two further occasions (T1 and T2) separated by 16 +/- 1 d. The protocol consisted of three 10-min stages of cycling at 45, 55, and 65% VO2max, followed by a final bout at 85% VO2max that was continued until exhaustion. Approximately 5 d after T1 the subjects were assigned, in a double-blind manner, to either phosphatidylserine (PS) or placebo (P). Breath-by-breath respiratory data and heart rate were continually recorded throughout the exercise protocol, and blood samples were obtained at rest, during the rest periods within the protocol (Post-55, Post-65), at the end of exercise (Post-85), 20 min after the completion of exercise (postexercise), and the day following exercise (Post-24 h).RESULTS: The main finding of this study was that supplementation had a significant effect on exercise time to exhaustion at 85% VO2max (P = 0.005). The exercise time to exhaustion in PS increased following supplementation (7:51 +/- 1:36 to 9:51 +/- 1:42 min:s, P = 0.001), whereas P remained unchanged (8:09 +/- 0:54 to 8:02 +/- 0:54 min:s, P = 0.670). Supplementation did not significantly affect oxygen kinetic mean response times (MRT(on) and MRT(off)), serum cortisol concentrations, substrate oxidation, and feeling states during the trial. CONCLUSION: This is the first study to report improved exercise capacity following phosphatidylserine supplementation. These findings suggest that phosphatidylserine might possess potential ergogenic properties.