Exercise is a potent stimulus for the release of human growth hormone (hGH), but the time course of the hGH response to sprint exercise has not been studied. The aim of the present study was to determine the time course of the hGH response to a 6 s and a 30 s maximal sprint on a cycle ergometer. Nine males completed two trials, on one occasion performing a single 6 s sprint and on another a single 30 s sprint. They then rested on a couch for 4 h while blood samples were obtained. Three of the participants completed a further control trial involving no exercise. Metabolic responses were greater after the 30 s sprint than after the 6 s sprint. The highest measured mean serum hGH concentrations after the 30 s sprint were more than 450% greater than after the 6 s sprint (18.5 +/- 3.1 vs 4.0 +/- 1.5 microg l(-1), P < 0.05). Serum hGH also remained elevated for 90-120 min after the 30 s sprint compared with approximately 60 min after the 6 s sprint. There was a large inter-individual variation in the hGH response to the 30 s sprint. In the control trial, serum hGH concentrations were not elevated above baseline at any time. It would appear that the duration of a bout of maximal sprint exercise determines the magnitude of the hGH response, although the mechanism for this is still unclear.