Fluctuations in the skin temperature of volunteers in water, of up to +/- 1.5 degrees C, were induced by circulating water through a tube suit at a fluctuating temperature for 80 min. Fluctuations with a cycle length of 2.5, 5.0 or 10.0 min all slowed the fall in core temperature that otherwise occurred in water at 29 degrees C, and reduced thermal comfort. Metabolic rate increased. Symmetrical fluctuations, or ramp fluctuations with either slow cooling and rapid warming phases or slow warming and rapid cooling phases, were all effective. Similar but less clear effects on core temperature were usually produced by fluctuating skin temperature in water at 25 and 33 degrees C; metabolic rate tended to increase during fluctuation in the 25 degrees C experiments, but not in the 33 degrees C experiments. Stability of the temperature of cutaneous receptors appears to be factor in insidious hypothermia produced by immersion in mildly cool water.