Effects of administering 10% ethyl alcohol as drinking fluid to mice during pregnancy and lactation have been examined by ethological analysis of behaviour of nursing females in their home cages at 1 day, 5--7 days, and 12--14 days postpartum. The treatment with alcohol did not affect gestation period or litter size, and fluid intake of treated mice remained similar to that of controls, the average intake of alcohol amounting to 29 mg/g body weight during lactation. Increase in frequency of exploration at 1 day postpartum was the only significant behavioural effect of alcohol on the nursing female mice. Duration of Non-Social Behaviour was unaltered, and no effects of the treatment on Maternal Behaviour or on Social and Sexual Investigation of male partners could be demonstrated. Behaviour of nursing females changed with increase in age of their pups. This occurred to a similar extent in treated and control animals. Maternal and Non-Social Behaviours declined in frequency as the pups became older although the time spent in these behaviours remained fairly constant. Social Investigation of the male partner declined both in frequency and duration while females were nursing their pups.