The oxygen consumption rates of virgin and lactating rats fed ad libitum were measured at temperatures ranging from 1 to 28 °C. The oxygen consumption rates of the virgin animals decreased progressively with increasing ambient temperature, but those of the lactating animals decreased slightly between 1 and 8 °C, remained approximately constant between 8 and 20 °C and fell rapidly above 20 °C. It is suggested that the fall in oxygen consumption rate above 20 °C in the lactating rats was due to a reduction in milk synthesis rate, and that the thermoneutral zone for normal lactation was 8–20 °C; this is a substantially cooler range than that of virgin rats (28–29 °C). The net energetic efficiency of milk production was calculated from the energy output in milk and the amount of energy used to produce milk (estimated from the oxygen consumption rates). The value obtained