One-to 5-day old Arbor Acre X Vantress chicks were exposed to a 10[degree] C ambient temperature and cloacal temperatures were recorded every 15 min. for 1 hr. Young chicks show a rapid development of thermoregulation the 1st 5 days after hatching, with significant changes apparent even during the first 1 1/2 day. This rapid homeothermic development is correlated with age and cannot be explained by increases in body weight, food intake, or insulation.