The behaviour of lactating mice in their home cages was examined by ethological procedures at 1, 7, 14 and 21 days postpartum. Early in lactation, maternal behaviour was more frequent in the light phase of the 24 hr cycle, whereas non-social activity occupied a greater amount of time during the dark phase. As the pups became older, maternal behaviour declined, and at 21 days the dams showed a marked increase of solitary immobility. Behavioural alterations produced by the administration of sodium valproate at 600 mg/l as drinking fluid during pregnancy and lactation (group SVP), and during lactation only, (group PN) were assessed. (Intake of drug amounted to 153 mg/kg during gestation and 186 mg/kg in lactation). Dams of group PN showed behavioural differences from controls in late lactation, pup nursing being prolonged at day 14 postpartum and the frequency of all categories of active behaviour, other than nursing and nestbuilding, was increased at day 21 postpartum in the dark phase of their daily cycle. There was not significant effect on categories of behaviour in dams of the SVP group. Overall, sodium valproate, at this dose, did not reduce maternal care.