The influence of exogenous oxytocin (OT) on maternal and infant behavior over 60 min of suckling, which followed 6 h of isolation, was investigated on Day 12 of lactation in rats. Mothers administered 1 IU of OT or saline through an indwelling atrial catheter and their litters indicated a similar nursing and suckling pattern, which was estimated by the crouching time of the mothers and the number of stretch reactions performed by the litters during a suckling period. To assess the alteration of the suckling intensity by OT administration, the plasma prolactin (PRL) level was determined by an Nb2 lymphoma cell bioassay. In the control group, the plasma PRL level increased and reached a peak at 45 min after the onset of suckling in 60% of the animals. The suckling-induced PRL release was completely inhibited and/or markedly delayed by OT administration. The difference in body weight of the litters before and after a suckling period was estimated as an index of the amount of milk suckled by the litters. There was no difference in the amount of milk between the control and OT-treated groups during a 60-min suckling period. However, it was significantly greater in the OT-treated group during the first 20 min of the suckling period. These results indicate that a dose of OT is a factor in the attenuation of the intensity of suckling done by the pups, whereas the nursing and suckling behavior is not influenced by OT administration.