Larval Ambystoma tigrinum were non-occlusively cannulated in the truncus arteriosus and allowed to recover 20-24 h. Upon recovery, control blood samples were removed, and the animals were exercised to exhaustion by encouraging swimming with a sponge forceps. The exercise led to a 5-6 fold increase in plasma lactate concentration which persisted 4-8 h. This lactacidosis caused the blood pH to drop from about 7.72 to below 7.10 in 30 min. Recovery was complete after 8 h. The acidosis was mixed with a pronounced respiratory component as PCQ more than doubled during the same period. Bicarbonate concentration decreased from 12 mM to about 6 mM following exercise and recovered over the following 8 h. Similarly, strong ion difference was depressed following exercise but recovered during the ensuing 8 h. When exercised animals were forced t o recover in running distilled water to deprive cutaneous ion transport mechanisms of access to ions, the results were quite different. The recoveries of pH, [HCOS-], and strong ion difference lagged 1-4 h behind the recoveries of these parameters in the tap water group, suggesting the participation of cutaneous ion transport in the recovery process. Ionic flux measurements demonstrated an increase in Naf influx during the recovery period as well as increases in both Naf and C1effluxes. The increases in efflux were much larger than observed increments in influx, placing the animals in negative ion balance and obscuring the significance of the changes in influx to acid-base balance. Measurement of circulating aldosterone revealed a large increase in the concentration of this steroid hormone during recovery.