Lactating Long-Evans rats were observed to interact differently with male and female pups during the first 18 days postpartum. Differences in the mother's behavior were related to the gender composition of her litter (GHL), to the sex of a single introduced pup, and to the sex of individual pups within her litter. Major differences were the greater time spent in licking the anogenital region of own male pups and the greater stimulation of anogenital licking by male foster pups, an effect that did not interact with GHL or age of pup. The GHL interacted with day of testing to affect nest building and time spent near pups.