Specimens of rainbow trout (Salmo gairdneri) were electrically stimulated to exhausting activity in a closed water recirculation system and the changes in dorsal aortic plasma pH, PCO2, PO2, O2 content, [Na+], [Cl-], [K+], [Lactate-] and Ht were measured during a 24 h recovery period. Net transfer of H+, Na+, Cl- and ammonia between fish and environment were determined by measurement of the concentration changes in the recirculating water. Strenuous exercise resulted in a severe lactacidosis which was corrected by transient net transfer of H+ ions to the environmental water within 4 h, about 6-8 h before the lactate was metabolically removed. The net transfer of H+ ions was achieved in part by branchial HCO3-/Cl- ion exchanges, but to a larger extent by branchial exchange of H+ and/or NH4+ against Na+. The excretion of ammonia, which was considerably enhanced during the first 4 h after exercise, was at least partially due to non-ionic diffusion across the gill epithelium. The observed elevation in ammonia excretion was probably the result of an exercise-induced increase in nitrogen metabolism rather than of production of ammonia for the purpose of acid-base regulation.