Brown adipose tissue (BAT) thermogenic activity has been measured in lactating rats on day 11 post-partum. In order to assess the response of this tissue to different rates of milk production, litter sizes were adjusted to give 2, 4, 8 or 12 pups/dam. In all lactating animals, BAT mass and protein content were markedly reduced, but thermogenic activity was only fully suppressed in dams nursing large litters. In those with 4 pups, guanosine diphosphate-binding activity (expressed as pmol GDP bound/mg mitochondrial protein) was lower than that of virgin control rats, but remained well above values obtained from control animals acclimated at thermoneutrality. Other estimates of BAT thermogenesis, cytochrome oxidase activity and noradrenaline-stimulated increase in oxygen consumption supported the inverse relationship between litter size and BAT function. Possible control mechanisms for the observed changes in BAT activity in lactating animals are discussed.