1. To investigate differences between the metabolic effects of light and heavy exercise, five healthy males (mean maximal oxygen intake 3.92 litres/min) exercised for 40 min at 36% maximum power (light work) and 70% maximum power (heavy work) on separate days, after an overnight fast. 2. A steady state was achieved in both studies between 20 and 40 min in: oxygen intake (1.42 and 2.64 litres/min respectively); respiratory exchange ratio (0.89 and 1.01); plasma lactate concentration (1.78 and 9.94 mmol/l). 3. Plasma palmitate turnover rate (14C) was unchanged from resting values in light work but was decreased by 40% (from 104 +/- 16 to 63 +/- 8 micronml/min) in heavy work. Heavy work was associated with falls in the plasma concentrations of all free fatty acids measured: palmitic acid (C16:0), oleic acid (C18:1), stearic acid (C18:0), linoleic acid (C18:2) and palmitoleic acid (C16:1). 4. In contrast to th fall in palmitate turnover the increase in plasma glycerol was greater in heavy exercise (0.054-0.229 mmol/l) than in light exercise (0.053-0.094 mmol/l), suggesting that lipolysis was occurring which did not lead to influx of free fatty acids into plasma. 5. In light exercise fat metabolism may be controlled to favour adipose tissue lipolysis and extraction of free fatty acids by muscle from the circulation, whereas in heavy exercise adipose tissue lipolysis is inhibited and hydrolysis of muscle triglycerides may play a more important part. 6. The finding of a high respiratory exchange ratio may not exclude the use of fat as a major fuel source in exercise associated with lactate production.