Forty experimentally heatstroke dogs were cooled by immersion in water at temperatures varying from 1-25 degrees C. At all water temperatures, cooling occurred at a much slower rate in comatose dogs in conscious ones. Dogs cooled in tap water (15-16 degrees C) had the same cooling rate as those cooled in ice water (1-3 degrees C). At water temperatures above 18 degrees C, cooling rates were considerably decreased. Since tap water cooled as efficiently as ice water, its use may prove the method of choice for human heatstroke victims. Its advantages are ready availability, simplicity of use, and its failure to cause shivering.