Squirrel monkeys controlled the air temperature within their test chamber by pulling a chain to select between two preset air temperatures, 10 and 50 degrees C. When the force required to pull the chain was increased in steps from 2.94 to 6.86 N, interresponse interval increased, resulting in wider air temperature swings within the chamber. The average air temperature selected became progressively lower, producing a concomitant fall in skin temperature. However, internal body temperatures (rectal and brain stem) remained nearly constant due to a compensatory increase in metabolic heat production. By allowing a cooler, rather than warmer, environment to result from a decreased response rate, the monkeys guarded against hyperthermia, which they are ill equipped to handle autonomically.