The rates of lipolysis and lipogenesis in adipocytes, isolated from biopsy samples of subcutaneous fat, was assessed by estimation of glycerol release during a 30-min incubation, and of the incorporation of 14C-glucose into lipids during a 1-h incubation at 37 degrees C, respectively. The subjects were six highly-qualified, active endurance sportsmen, eight former endurance sportsmen of international class, and six untrained young men. In the active sportsmen the basal rate of lipolysis was about half of that in the previously-active sportsmen and the untrained subjects, but after the addition of adrenaline (10(-4) or 5 x 10(-4) mol.l-1) the lipolysis rate was the highest. No differences were observed in the lipolytic rates in the former sportsmen compared to the untrained subjects. Gases of a comparatively high level of lipogenesis were found in the trained subjects. The addition of insulin (9 microU.ml-1) to isolated adipocytes caused a significant augmentation of individual rates of lipogenesis in the active sportsmen and the untrained persons but not in the previously-active sportsmen. In comparison with the active sportsmen, the previously active sportsmen revealed an increased basal rate of lipolysis and a reduced sensitivity to the lipogenic action of insulin. These findings suggest that these changes may have had significance in avoiding an increase of adipose tissue after a decrease in energy expenditure due to a change in physical activity.