Previous findings that rats reared in isolation following a 3-week nursing period ultimately show a decrement in response to the presence of other rats were extended in the present study. The data suggested that (a) not only understimulation (isolation) but overstimulation (crowding) can reduce social interaction, (b) pairings containing cage mates (high acquaintanceship) can increase social interaction, (c) age and steepness of autonomic-reactivity habituation gradients are inversely related, (d) male rats, in comparison with female rats, are more autonomically reactive in mixed physical-social environments as has been found in purely physical environments, and (e) male rats, in comparison with female rats, are more sensitive to the effects of deviations from normal postweaning social stimulation in relation to both social and autonomic reactivity in the open field.