the effects of nicotine administered during one of three gestational trimesters on development and behavior of rats were investigated. pregnant female shs (satinder's heterogeneous stock) rats were injected subcutaneously with nicotine (0.0/, 0.3 or 0.6 mg/kg/day) during one of three gestational trimesters (days 0-6/ days 7-13/ or days 14-20). no differences were found between groups in litter size/ litter weight/ male-female litter ratio or postnatal mortality. however/ progressively later trimesters and higher nicotine dosages retarded the appearance of developmental signs and' reflexes. at 29 days of age/ one day after weaning/ emergence from home cage was measured. rats prenatally exposed to 0.3 mg/kg/day of nicotine and rats exposed during the second trimester (7-13) had shorter emergence latencies than the other groups. following emergence testing/ all animals were exposed to the open field for 4 successive days. the 5-minute session each day revealed higher defecation scores and increased activity in the group prenatally exposed to 0.3 mg/kg/day of nicotine. animals prenatally treated during the third trimester/ defecated significantly more than animals treated during the first trimester. on completion of open field testing#, each rat was examined for an unconditioned escape response to electric shock. no differences were found between groups when litter differences were taken into account. after one day of rest#, animals were given 4 days of either-way avoidance training. following training# the effects of postnatal nicotine administration were assessed by administering nicotine (0.0# 0.2# 0.3 or 0.4 mg/kg counterbalanced over 4 days) 25 minutes before avoidance testing. rats prenatally exposed to nicotine during the second trimester displayed superior avoidance acquisition and avoidance performance during the postnatal nicotine challenge#, but this difference became non-significant when the differences between litters were considered. differences in one-way avoidance performance with the different postnatal nicotine doses were only observed for two groups: animals exposed to saline during the first trimester and animals exposed to 0.3 mg/kg/day during the third trimester. in both of these groups# the postnatal nicotine dose most comparable to the dose received prenatally resulted in the best avoidance performance. the effects of pre- and postnatal nicotine on development and behavior and the presence of greater susceptibility to a specific nicotine dose or gestational period is discussed. it can be concluded that different nicotine dosages administered during different periods of gestation can differentially affect development and behavior.