Infant voles thermoregulate poorly and produce ultrasonic vocalizations when cooled. Vocalizing and the ability to maintain body temperature in isolated pups cold-challenged at 5 degrees C or 22 degrees C were studied in nestling Clethrionomys glareolus, Microtus agrestis, and Arvicola terrestris. The tendency to vocalize varied with age, since pups vocalized more in their 2nd week than in their 1st or 3rd weeks. Rate of vocalizing was correlated with sound pressure level of vocalizations. Their was no apparent relation between vocalizing rate and deep body temperature. M. agrestis pups vocalized most and A. terrestris pups least, and all three species vocalized more at the lower temperature. Maximal vocalizing occurred in mid aged M. agrestis (at 5 degrees C) with mean of 1291 vocalizations/20 min and mean SPL of 80 dB (decibels re: 20 microN/m2). It is suggested that the vocalizing response is an adaptation related to risk from hypothermia in infant voles.