SUMMARY1. Thermoregulation against cold exposure was studied in rats during pregnancy and early lactation, and compared with that of virgin rats.2. When exposed to 0 TC for 60 min, rats which were within 24-48 h of parturition (pre-1-day rats) and those within 24 h of parturition (pre-0-day rats) showed significantly larger falls of colonic temperature (To) than virgin rats. The temperature decrease was greatest in the pre-0-day rats, being 4' 1 + 0 4 0C (mean + S.E.M.) at the end of cold exposure, compared with a decrease of 1-7 + 0-3 0C in the virgin rats. The tail skin temperature fell to 0 0C during cooling in all virgin rats and in pregnant rats at each gestational stage.3. During cold exposure at 10 0C for 30 min, pre-0-day rats also showed significantly larger falls in Tco (1I8+006 'C) than virgin (0-4+0-2 'C), pre-1-week (0-8 + 0 3 0C), post-0-day (0 3 + 0 3 0C) or post-I -week rats (0 4 + 0 3 0C). Although body weights in pre-0-day rats were far larger than those in virgin rats, the increase in oxygen consumption per animal during cold exposure was 50 % lower in pre-0-day rats (2-2 + 0 5 ml/min) than in virgin rats (5 3 + 0-3 ml/min). There was no difference in basal oxygen consumption per animal between the late pregnant and virgin rats.4. Within 24 h after parturition, both the decrease of Tco and the increase of oxygen consumption during cold exposure returned to the values observed in virgin rats.5. The present results demonstrate clearly that cold-induced thermogenesis is significantly suppressed in rats at a late stage of pregnancy.