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The effects of differential preweaning social isolation of rats during Days 17-22 on the development of muricide, open field behavior, and starvation stress tolerance were examined. Four weaning isolation groups were used in order to separate the effects of maternal and littermate isolation during this period. EMLI was deprived of maternal and littermate contact; EMI, maternal contact; and ELI, littermate contact. NWI rats were isolated on the day following normal laboratory weaning. The results indicated a marked decrease in muricide in response to maternal isolation (EMLI and EMI) and a slight decrease in response to littermate isolation. In the open field the maternally isolated rats were more active than the other groups and exhibited less defecation. There were, however, notable differences between EMLI and EMI. EMLI was more active than EM1 in the open field and exhibited an increase in activity over the 4 testing periods while EMI, ELI, and NWI decreased activity. EMLI was clearly hyperactive during muricide testing while EM1 was the least active. EM1 rats weighed less than those in the other groups on Days 35,50, and 100 but survived terminal food deprivation the longest. Significant differences in nonstressed adrenal gland weight were observed among groups but did not appear to correlate with any of the other measures. Terminal starvation eliminated these differences. Starvation also induced muricide in nonkillers in each of the groups. NWI rats appeared to be most easily converted and exhibited more consistent muricide than the other groups. These results are discussed in terms of a theory of altered forebrain regulation of emotional reactivity in response to preweaning stress and isolation.Studies examining the effects of manipulating the social environment of rats during neonatal development indicate that the weaning period is a "critical period" for physiological and behavioral development. Premature weaning appears to be a major form of stress and results in permanent alterations in neuroendocrine regulation

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