Five healthy males performed four 30-s bouts of maximal exercise, separated by 4 min of rest, on an isokinetic cycle ergometer. Arterial blood and urine samples were taken from indwelling catheters at rest, immediately postexercise, and for 90 min of recovery. Inulin was continuously infused to measure glomerular filtration rate (GFR). Arterial plasma [Na+], [K+], and [Cl-] increased (P less than 0.05) with exercise; plasma lactate concentration ([Lac-]) increased from 1.3 +/- 0.2 to 21.0 +/- 1.0 (SE) meq/l (P less than 0.05). A significant decrease in the GFR occurred after exercise and during recovery associated with reductions in renal Na+ and K+ excretion (P less than 0.05). Renal excretion of Lac- reached a maximum of 293 +/- 79.4 mu eq.kg-1.h-1 (P less than 0.05), with Cl- excretion reaching a minimum of 4.8 +/- 0.95 mu eq.kg-1.h-1 (P less than 0.05). Urine [Lac-] was 189 +/- 25.6 meq/l, and urine [Cl-] was 6 +/- 1.7 meq/l at 30 min of recovery. There was a curvilinear relationship between urine [Cl-] and [Lac-] (r = -0.86; P less than 0.0001). Net Lac- production was estimated from arterial [Lac-] and after assuming a distribution volume. Less than 2% (13.1 meq) of the total estimated Lac- produced (678 meq) was excreted in the urine. Decreases in urine [Cl-] act to limit the fall in urine pH that accompanies increases in urine [Lac-].