The purpose of this investigation was to determine the effect of moderate weight lifting (MWL) and light weight lifting (LWL) on concentrations of serum testosterone in males. Baseline testosterone concentrations were determined via an indwelling catheter in the median cubital vein. An initial sample of blood was followed 7 min later by five samples taken at 4-min intervals. A final sample was taken 10 min after the last 4-min blood draw. Blood samples were obtained at similar times and intervals on the weight lifting days. The MWL consisted of four sets of six squats at 90-95% of a six-repetition maximum (RM), while the LWL consisted of four sets of 9 or 10 repetitions at 60-65% of the weight used for the sets during MWL. There was a significant increase in serum testosterone concentrations following the fourth set for both MWL and LWL when compared with baseline concentrations and both MWL and LWL testosterone concentrations returned to baseline levels at 10 min postexercise. These results indicate that MWL and LWL caused increases in serum testosterone that were greater than those associated with baseline levels. Postexercise responses for the MWL and LWL were similar.