SUMMARYRed cell membranes were isolated from blood samples obtained from athletes during exhaustive exercise on a bicycle ergometer and during the subsequent recovery period of 60 min. Plasma lactate levels were also determined. During exercise, cell membranes were progressively depleted of cholesterol and, at exhaustion, membrane cholesterol was less than 80 % of the initial level. A parallel decline in Na+, K+-ATPase was also noted, while phospholipid reduction was around 5 %. During recovery, the erythrocyte membrane cholesterol and Na+,K+-ATPase increased, but at a slow rate and were inversely proportional to plasma lactate content.