The acute, transient effects of moderate exercise on plasma lipoprotein parameters were assessed in 12 healthy, normolipidemic male volunteers subjected to a 5.5-km run at a comfortable pace. The course was completed in 31.5 min (range 23-43 min) and elicited a significant increase in pulse rate and plasma lactate levels and a fall in diastolic blood pressure. Of the plasma lipid parameters, total phospholipid, apo B-associated phospholipid, plasma triglyceride, and free fatty acid levels increased substantially immediately after the run and, except for triglyceride, were still raised 2 h later. Lesser but also significant increases were also noted in high-density lipoprotein-cholesterol (HDL-C), HDL3-C, and HDL-phospholipid concentrations after the run. The increase in HDL-phospholipid persisted for 2 h but the others returned to basal levels. Apolipoprotein Al, lecithin: cholesterol acyltransferase activity, total cholesterol, low-density lipoprotein-cholesterol, and the free cholesterol:cholesterol ester ratio remained essentially unaltered by exercise. The results were compatible with an increased secretion of triglyceride and phospholipid-rich VLDL, followed by the rapid clearance of the triglyceride moiety from the plasma and some redistribution of surface components to the HDL fraction.