Eight young men underwent a programme of training by running for 30 min at moderate speed three times a week for 4 weeks. Metabolic and hormonal changes in blood were studied during and after a run of 60 min at comparable speeds before and at the end of the training programme. Increases in lactate, pyruvate and plasma FFA during exercise were less after training. Increases in glucose were greater. There was a smaller increase in the post-exercise concentration of blood ketone-bodies after training. Plasma levels of insulin and human growth hormone (HGH) were lower after training. The fall in insulin and the rise in HGH during exercise were also smaller. There is a dissociation of the normal relationship between blood glucose and insulin during exercise. Insulin appears to be more important in the control of fat metabolism, in which its role may be altered by physical training. The changes observed in the longitudinal study of training imply that differences observed in cross-sectional studies of athletes and untrained subjects are not the result of an innate difference but do depend upon metabolic changes related to athletic training.