Major improvement in the homeothermic ability of the rat occurs during the first 2 weeks of postnatal development. Changes in thermoregulatory responsiveness to a single injection of ethanol (EtOH) may occur during this period immature rats (2‐3, 8‐9, and 14‐15 days of age) were administered either saline or EtOH (2 or 4 g/kg BW ip) at thermoneutral ambient temperatures (Ta). In one experiment, metabolic rate (MR) and body temperatures (colonic and skin) were recorded for 1‐3 hr postinjection. A second experiment determined blood EtOH concentration in rats from the 3 age groups over an 8‐hr period following injection of EtOH. 4 g EtOH/kg produced few significant reductions in thermoregulatory function of 2‐3 day‐old rats, but decreased MR by 16% and colonic temperature by 0.5‐0.7OC in 8‐15 day‐old animals. 2 g EtOH/kg had no effect on 8‐9 day‐old rats, but reduced MR and colonic temperature in rats aged 14‐15 days. In every case, the hypothermic response to EtOH was correlated with a reduction in MR. Back and abdominal skin temperatures decreased with colonic temperature, and tail skin temperature indicated EtOH‐induced vasoconstriction in older rats. Blood EtOH concentrations were similar in the three age groups during the first 2 hr postinjection and did not explain differences in metabolic response. The magnitude and duration of thermoregulatory responsiveness to EtOH increases w