The objective of this study was to investigate the potential causes of high body temperature (Tb) during lactation in mice as a putative limit on energy intake. In particular we explored whether or not offspring contributed to heat retention in mothers while suckling. Tb and physical activity were monitored in 26 female MF1 mice using intraperitoneally implanted transmitters. In addition, maternal behaviour was scored each minute for 8 h d−1 throughout lactation. Mothers that raised larger litters tended to have higher Tb while nursing inside nests (P < 0.05), suggesting that nursing offspring may have influenced heat retention. However, Tb during nursing was not higher than that recorded during other behaviours. In addition, the highest Tb during the observation period was not measured during nursing behaviour. Finally, there was no indication that mothers discontinued suckling because of a progressive rise in their Tb while suckling. Tb throughout lactation was correlated with daily increases in energy intake. Chronic hyperthermia during lactation was not caused by increased heat retention due to surrounding offspring. Other factors, like metabolic heat produced as a by-product of milk production or energy intake may be more important factors. Heat dissipation limits are probably not a phenomenon restricted to lactation.