Vasotocin was recently reported to increase the neonatal amount of active sleep in kittens. In this study we examined the effects of arginine vasotocin (vasotocin), given intraperitoneally and at a wide dose range, on the sleep-wake behavior of 7-day-old and 14-day-old rat pups using a static charge sensitive recording system. It increased the percentage of quiet state relative to total sleep time at the doses of 0.01-1.0 ng/g of body weight but did not affect it at the dose of 0.001 ng/g. The percentage of active sleep was decreased by doses of 0.01-10.0 ng/g. The increase in the amount of quiet state and the concurrent suppression of active sleep lasted for 6 hours. Wakefulness was slightly increased at 10 ng/g of vasotocin. These findings indicate that, in contrast to the reports on kittens, in newborn rats peripheral administration of vasotocin increases the amount of quiet state relative to total sleep time and decreases that of active sleep.