It is known that the concentrations of ammonia and lactate in blood increase during incremental exercise. Sweat also contains lactate and ammonia. The aim of the present study was to investigate the physiological response of lactate and ammonia in plasma and sweat during a stepwise incremental cycle ergometer exercise test in ten subjects. During this test lactate and ammonia were measured in blood obtained from the earlobe and in sweat collected in a bag attached to the back of the subject. At the end of each interval this bag was emptied for measuring lactate and ammonia. A disproportional increase in the concentration of lactate and ammonia in blood was found, in sweat a disproportional decrease. The lactate concentrations in sweat were higher than those in blood. We hypothesise that lactate in sweat is produced from glycogen granules of the clear cell of the eccrine gland. This lactate production results in acidification of sweat, which facilitates the diffusion of ammonia from eccrine duct cell to duct lumen. It is uncertain how far duct cell ammonia originates from plasma, the duct cell itself might produce ammonia. Part of the ammonia in sweat could come from the breakdown of urea by skin bacteria.