In 2-week-old rats, isolation from social companions in an unfamiliar place elicits ultrasonic vocalization rearing and locomotor behavior. These responses are prevented and close body contact is maintained if littermates or the dam are present. It has generally been believed that such isolation distress and comfort responses develop as a result of prior social experience. We reared pups in isolation from the third postnatal day and found essentially normal responses. A second experiment replicated and extended these results to pups reared in isolation from the day of birth. Isolation-reared pups did have higher levels of ultrasonic vocalization than their normally reared littermates, lower body weight, and lower post-test temperatures. We conclude that a lengthy period of close body contact and frequent social interactions with conspecifics are not necessary for the development of attachment responses in this species.