Chronic endurance exercise training has been associated with decreased levels of total cholesterol and increased HDL-cholesterol. To our knowledge rapid changes in cholesterol and HDL-cholesterol during acute exercise have not been described under controlled conditions. We studied 28 subjects (14 males and 14 females) during bicycle exercise for 40 min at a work intensity of 55% of their maximal oxygen consumption. Total and HDL-cholesterol levels were measured (and LDL-cholesterol calculated) at rest, 10, 20, 30, and 40 min of exercise, and 15 min postexercise. There was a significant (p less than 0.001) increase in HDL-cholesterol levels at 10 min of exercise (58.8 +/- 13.9 mg/dl, mean +/- SD) above rest (53.1 +/- 13.4 mg/dl) for all subjects. This increase persisted (p less than 0.001) at all time points throughout the exercise session, but declined by 15 min postexercise. There was a small, insignificant decline in LDL-cholesterol. It is concluded that apparent favorable changes in lipoprotein patterns occur acutely, and are sustained during short-term, moderate intensity exercise. Analyses of these changes appears necessary if the biochemical mechanisms which underlie these metabolic alterations are to be elucidated.