The effect of acute exercise at three graded intensities on plasma growth hormone (GH) and prolactin (PRL) concentrations was examined in three groups of healthy male volunteers. According to their training status these subjects were divided into untrained, moderately trained and highly trained. A clear response of GH to exercise was registered already at an intensity of 50% of maximal oxygen uptake (VO2max) with a maximal response at 70% VO2max and no further effect at 90% VO2max. In contrast, no PRL response was observed at 50% VO2max, a small PRL rise was seen at 70% VO2max and the highest response occurred at 90% VO2max. Basal and exercise-stimulated plasma GH and PRL concentrations were similar in the three groups tested at similar relative workloads, suggesting that physical training induces adaptive changes whereby higher absolute workloads induce similar hormonal and metabolic changes. To examine a potential causative role of lactate in inducing the GH and PRL responses, sodium L-lactate was infused intravenously to normal sedentary volunteers at doses producing plasma lactate concentrations within the range of those seen between 70 and 90% VO2max. This resulted in a significant elevation of plasma GH and PRL concentrations, which, however, were smaller than those obtained at an exercise-induced matched plasma lactate concentration. We conclude that physical training causes adaptive changes in highly trained runners so that identical GH and PRL responses to exercise are recorded at higher absolute workloads. Lactate may be involved in the exercise-induced GH and PRL response; however, it does not appear to play an exclusive role.