Limits to sustainable metabolic rate were sought in lactating MFI mice, Mus musculus. Lactating mice appeared to reach a limit in the food intake during later lactation (days 13 to 16) at 23 g/day, and also with litter sizes above 8 pups. This plateau resulted in pups from larger litters being lighter than pups from smaller litters. Maternal resting metabolic rate (RMR) increased two fold from prebreeding to peak lactation. Prebreeding RMR was not found to be correlated with either litter size or litter mass. RMR at peak lactation was positively correlated with litter size, but when the effect of body mass on both RMR and litter size was removed, the relationship was no longer significant. The major morphological component influencing RMR at peak lactation was the mass of the carcass and skeletal muscle. When females faced the demands of pregnancy and lactation concurrently, they ate significantly less food and yet raised heavier pups, than females just lactating. To avoid the peak demands of pregnancy and lactation overlapping, females delayed the implantation of the concurrent pregnancy by between 2 and 11 days. The extent of this delay was related to the size of the lactating litter. Lactating females exposed to the cold (8^oC) for the second half of lactation increased their food intake by an average of 30%, however the pups were still lighter than those raised at 21^oC, and fewer were weaned in the cold. Lactating females were unable to increase their milk energy output, with increasing litter size at 21^oC. There was no difference in the gross energy content of the milk produced by females at 21^oC or at 8^oC. However, the females raising litters in the cold produced a greater volume of milk than those in the warm. The results obtained indicate that although females reach a plateau in their food intake and milk production with increasing litter size, this may not be due to a limit acting either on the alimentary tract or the mammary tissues, as females are able to increase both food intake and milk production when exposed to cold during lactation.