OBJECTIVE: To evaluate the impact of repeated neonatal mother deprivation (RMD) of male rats on the behavioral parameters and response of prolactin to mild stress stimuli in the adulthood. METHODS: Afer birth, the pups of Wistar Porton Olac rats were crossfostered and their number was adjusted to 8 per litter (4 males and 4 females). They were removed from the dam for 6 hours daily on postnatal day 6, 7, 8 and for 12 hours daily on postnatal day 12, 13, 15, 16 and placed to another cage lined with cotton wool at controlled temperature 37 øC. Body weight was estimated repeatedly from postnatal day 9 to 97. At 14 weeks of life the behavioral activity was measured in an open field on 2 occasions, 2 days apart. One week later the rats were exposed to 15 min novelty stress or to 3 min handling and decapitated 15 min after the initiation of both. Trunk blood was collected and plasma prolactin (PRL) was measured by radioimmunoassay. RESULTS: On postnatal day 15 the eye opening was found in 75 % of control pups and 73 % of pups with RMD. In the rats after RMD the body weight gain was significantly decreased from day 21 until the day 97. Vertical behavioral activity (rearing) was enhanced in RMD rats when measured on the first occasion. Horizontal behavioral activity did not significantly differ from the control group. Stress of novel environment elicited the activation of PRL secretion in untreated animals (19.3 1+4.6 ng/ml vs. 7.17+1.03 ng/ml, P<0.05), while no change was found in the rats after RMD (8.15+2.0 ng/ml vs. 4.35+0.48 ng/ml). CONCLUSIONS: In the rats exposed to neonatal mother deprivation the lower emotionality was found. Significantly decreased body weight gain in these animals was probably due to the nutritional deprivation during the postnatal separation from the mother. The nonresponsiveness of lactotrophs to mild stressor in adult rats after RMD may have a negative impact on defense mechanisms to immune challenges.