Evidence suggests that acute exercise and endurance training has a suppressive effect on the hypothalamic-pituitary-gonadal (HPG) axis in men and women. To determine if training and acute exercise influence the neuroendocrine regulation of the HPG axis in men we examined pulsatile LH release in six male endurance runners with a training volume of at least 80 km per week, and compared this with values in six age-matched sedentary controls. Blood samples were obtained through an indwelling i.v. cannula from the subjects at 15-min intervals for 6 h following 24 h without significant physical activity and again in the runners, following 60 min of running at a speed equivalent to 5% below the anaerobic threshold. Mean LH pulse frequency and amplitude, as well as areas under the LH pulses and total LH curve, were calculated but only the mean post-exercise area under the total LH curve area was significantly lower than basal values (P less than 0.05) following exercise compared with the resting values in runners. Other measures of LH release did not change with acute exercise. Basal and pre-exercise testosterone levels were also measured and found to be at the lower end of normal men. The mean pre-exercise serum testosterone levels were significantly higher than basal levels. Mean testosterone levels, mean pulse amplitude, and mean area under the LH curve were significantly lower in resting runners than in the controls. The data suggest that exercise induces a general lowering of LH levels but does not inhibit LH pulsatile release. An anticipatory increase in serum testosterone occurred before exercise.