Arginine-vasopressin (AVP) is involved in memory processes. The memory effects of AVP are mediated by neuronal mechanisms taking place in limbic-midbrain structures. Therefore, immunoreactive AVP (IR-AVP) was measured in hippocampus and amygdala of male Wistar rats during acquisition and retention of passive avoidance behavior. IR-AVP concentration was decreased in the hippocampus immediately after the learning trial while IR-AVP content of the amygdala was not affected. Animals that showed the passive avoidance response (good avoiders) at the 24 h or 120 h retention test had a reduced IR-AVP concentration in the hippocampus immediately after the test. However, IR-AVP content of the hippocampus was not different from that of non-shocked control animals when measured immediately before the 120 h retention test. Poor avoiders that showed only minor avoidance behavior did not differ in hippocampal IR-AVP content from non-shocked control animals. IR-AVP content of the amygdala was also not altered after the retention session. These effects on IR-AVP content could only be shown in animals that were trained and habituated to the passive avoidance procedure. Such trained and habituated animals had an IR-AVP level in the hippocampus which did not differ from that of animals that were left undisturbed until sacrifice. When the animals were not trained, but placed for the first time in the passive avoidance apparatus without being exposed to the learning trial, the hippocampal IR-AVP content was reduced.(ABSTRACT TRUNCATED AT 250 WORDS)