It is purported that supplementation with Cordyceps Sinensis (CordyMax Cs-4) will improve oxidative capacity and endurance performance. The intent of this investigation was to examine the effects of CordyMax Cs-4 supplementation on VO<(2peak,) ventilatory threshold, and endurance performance in endurance-trained cyclists. Twenty-two male cyclists participated in 5 weeks of supplementation with CordyMax Cs-4 tablets (3 g/d). Training intensity was maintained by weekly documentation and reporting throughout the 5-week period. Subjects completed a VO(2peak) test and work-based time trial prior to and following the supplementation period. VO(2peak) was similar within and between placebo (PLA) and treatment (CS) groups prior to (59.9 +/- 5.9 vs. 59.1 +/- 5.4 ml/kg/min, respectively) and following (60.1 +/- 5.5 vs. 57.1 +/- 5.8 ml/kg/min, respectively) the supplementation period. Ventilatory threshold (VT) was measured at 72 +/- 10% of VO(2peak) in P and T prior to supplementation and did not change in either group following the supplementation. PLA completed the time trial in 61.4+/- 2.4 min compared to 62.1+/- 4.0 min in T. Time trial measurements did not differ between groups, nor did they change in response to supplementation. It is concluded that 5 weeks of CordyMax Cs-4 supplementation has no effect on aerobic capacity or endurance exercise performance in endurance-trained male cyclists.