The effects of four months' physical exercise on serum lipids, lipoproteins and lipid metabolizing enzymes were studied in 25 non-insulin-dependent diabetic patients divided randomly into exercise (n = 13) and control (n = 12) groups. Exercise induced a significant decrease in serum LDL-cholesterol and an increase in serum HDL-cholesterol and HDL2-cholesterol. Triglycerides showed a temporary decrease. Apoproteins A1 and B were virtually unchanged. Postheparin plasma lipoprotein lipase increased markedly during the exercise period while no change occurred in adipose tissue lipoprotein lipase, hepatic lipase or lecithin:cholesterol acyltransferase. In the control group no significant changes occurred in any of the lipid variables. In the light of the knowledge of LDL-cholesterol as a causative and HDL-cholesterol as a protective factor in atherogenesis in non-diabetics the changes caused by exercise in non-insulin-dependent diabetics can be considered favourable.