Effects of temperature and reproductive experiences on the energy budget of striped hamsters (Cricetulus barabensis) during lactation were examined in this study. The hamsters were exposed to a gradual decrease in the ambient temperature, from 30℃ to 0℃ throughout three bouts of lactation. The temperature was decreased from 30℃ to 20℃ during the 1^st lactation period, and similarly, from 20℃ to 10℃ and from 10℃ to 0℃ during the 2^nd and 3^rd lactation periods, respectively. For each bout, the energy intake and energy expenditure of female hamsters were measured during the peak lactation. Furthermore, resting metabolic rate (RMR) and nonshivering thermogenesis (NST) were quantified using an open-flow respirometry system, to indicate the rate of oxygen consumption. Cytochrome c oxidase (COX) activity of brown adipose tissue (BAT) was determined polarographically with oxygen electrode units. Serum thyroid hormone (tri-iodothyronine, T₃ and thyroxine, T₄) and prolactin concentrations were determined using a radioimmunoassay method. Total body fat was extracted from the dried carcass by ether extraction in a Soxhlet extractor. Body mass of female hamsters decreased significantly during lactation, and body mass measurements between days 7 and 17 of lactation decreased by 12.4%, 12.3%, and 13.5% in the 1^st, 2^nd, and 3^rd lactation groups, respectively. The sustained energy intake during the peak observed in the 3^rd lactation group was significantly higher than that in the 1^st and 2^nd lactation groups. Litter sizes were 4.4 ± 0.1, 4.4 ± 0.2, and 4.3 ± 0.4 in the 1^st, 2^nd, and 3^rd lactation groups, respectively, among which no statistically significant differences were observed. In contrast, significant differences in litter mass were observed among the three groups. Litter mass in the 1^st and 2^nd lactation groups decreased to a greater extent than in the 3^rd lactation group by 15.1% and 18.2%, respectively, on day 7 of lactation, and by 24.5% and 18.6%, respectively, on day 17 of lactation. Significant correlations were observed among the sustained energy intake, litter size, and litter mass. RMR and NST were also significantly different among the three groups, with the females in the 2^nd and 3^rd lactation groups showing higher RMR and NST than those in the 1^st lactation group. The sustained energy intake during peak lactation was positively correlated with RMR and NST. Neither wet nor dry carcass mass showed significant differences among the three lactation groups. Fat content in the 2^nd and 3^rd lactation groups decreased to a greater extent than that in the 1^st lactation group, by 12% and 28%, respectively. BAT COX activity in the 1^st and 2^nd lactation groups increased to a greater extent than in the 3^rd lactation group, by 139.7% and 65.5%, respectively. Serum T₃ concentrations were significantly higher in the 3^rd lactation group than in the 1^st lactation group, whereas the differences in T₄ concentration were not significant among the three groups. Serum prolactin concentrations also did not show significant differences among the three groups. These results suggest a trade-off between the energy allocation to thermogenesis and offspring, in female hamsters under successive decreases in temperature. Cold exposure induces a significant increase in thermogenesis but a decrease in reproductive output. Furthermore, striped hamsters may perceive a decline in ambient temperature and consequently decrease reproductive output. The findings of the present study are consistent with the prediction of "the seasonal investment hypothesis."