The clinical picture and biochemical changes reported in human victims of near-drowning is considerably different from those reported in dogs subsequent to total immersion in fresh water. To gain further insight into this apparent discrepancy and to study the differences in the pathophysiological response of animals to drowning in chlorinated fresh water compared to unchlorinated fresh water and isotonic saline solution, a group of experiments was undertaken.The changes in cardiovascular dynamics, blood constituents and serum electrolytes observed in this study were transient, frequently limited to the arterial samples; these spontaneously reverted to normal in the thirteen animals who survived the experiment. Ten to sixty minutes post-immersion the three groups studied were indistinguishable. Conversely, acute asphyxia with arterial hypoxemia and metabolic acidosis which persisted throughout the experiment was seen in all animals. The presence of chlorine in distilled water did not significantly alter the response to aspiration.