Five normal male volunteers performed two intermittent weight lifting exercises of equal total external work output and duration (20 min) with identical work-rest intervals but different load and frequency of movements. Exercise I consisted of seven sets of seven vertical leg lifts at 85% of the subject's Seven Repetition Maximum (SRM) and, 5 days later, seven sets of 21 vertical leg lifts with one-third of the previously used load (Exercise II). Blood was sampled throughout the exercise and recovery periods for growth hormone, lactate, and glucose analysis. Growth hormone increased after 20 min of Exercise I to a peak during the recovery period. Significantly elevated growth hormone (GH) levels were found 5, 10, and 15 min (P<0.025,P<0.05,P<0.025 respectively) of recovery after Exercise I. No significant elevations of GH occurred in Exercise II. Significant linear correlations (r=0.99,P<0.01) with a time lag of 16 min were found between lactate and GH levels in Exercise I (lactate increases preceded those of GH). No significant differences in plasma glucose concentrations were detected. The results suggests that in intermittent weight lifting exercises of equal total external work output and duration as well as identical work-rest intervals, the load and/or frequency of an exercise are determinant factors in the regulation of plasma GH levels.