The effect of 30 or 40 min hard exercise on a cycle ergometer upon plasma concentrations of zinc, iron and copper in twelve healthy male athletes was studied. Sweat samples were also collected from different body sites and analyzed for these metals. The metal content of sweat from different body sites varied, as did the sweating rates of different subjects. Pre-exercise plasma iron concentrations in 10 of the 12 subjects were within the normal range, but at least 4 subjects had sub-normal plasma zinc whereas six had plasma copper levels above normal. The effects of exercise on plasma metal concentrations varied from subject to subject; no general conclusions could be drawn about the biological significance of loss of metals in sweat in relation to whole body metal metabolism. It is suggested that a mild acute phase response may account for lowered zinc and raised copper in the plasma of athletes.