Stim-O-Stam is a commercial ergogenic aid purported to improve human performance by reducing recovery time and enhancing endurance. The tablets contain a mixture of sodium acid phosphate and potassium phosphate. In this study, under a double-blind cross-over design, 11 male subjects underwent acute (1.24 g one hour before exercise) and chronic (3.73 g X d-1 for 6 d prior to exercise) consumption of Stim-O-Stam to determine the effects of phosphate loading on: treadmill endurance time; treadmill endurance time after 15 min of recovery from first treadmill run; leg power measured for 1 min on the Cybex device; leg power after 10 min recovery from first power test; and oxygen uptake during the treadmill run. Run times ranged from 172 to 183 s on run 1 and 145 to 152 s for run 2. Leg power averaged 62 W for both tests. Oxygen uptake averaged 52 ml X kg-1 X min-1 under all conditions. Analysis of variance showed that there was no significant effect of acute or chronic ingestion of phosphate tablets beyond that of the placebo treatment on any of the performance tests. These results argue against the claim of ergogenic benefits ascribed to this nutritional supplement.