The delayed lipoprotein changes after a 3-hour running test were examined in 14 moderately trained young male subjects. Fasting blood samples were obtained one day before, immediately before, and one, two, and four days after the race. Nonfasting samples were collected immediately after, one, and three hours after exercise. Three hours after the race, the ratio of unesterified cholesterol to cholesteryl esters was significantly increased, and one and two days after the race it was significantly decreased compared to the preexercise value. The HDL2/HDL3 ratio, measured by density-gradient ultracentrifugation was one and three hours after the running significantly elevated. However, no redistribution of the HDL2/HDL3 cholesterol ratio determined by a precipitation method with polyanions was found at this time. One day postexercise HDL cholesterol rose significantly above the preexercise value, and this was associated with an elevation of the HDL3 subfraction. On the following day a significant increase of HDL2 cholesterol and the HDL2/HDL3 cholesterol ratio was found. The apolipoproteins A-I, A-II, and B, measured by radial immunodiffusion, did not change during the first hours and the first two days after the race. On the second postexercise day the Lp(a) lipoprotein rose significantly above the preexercise value. Compared with the preexercise level the LCAT activity was significantly elevated three hours after the race and significantly decreased two days later. The present study suggests that during the first few days after prolonged exercise a number of plasma lipoprotein changes take place that are similar to those observed after a period of physical training.