The relation between passive avoidance behavior and primary antibody response was studied in rats. The avoidance latency was varied by means of variation of the shock intensity during the learning trial; a higher shock intensity resulted in a longer avoidance latency at the retention test. The avoidance latencies were inversely proportional to the magnitude of the primary antibody response. Neuropeptides, such as [Arg8]-vasopressin (AVP) were shown to potentiate retrieval processes. Administration of 10 ng desglycinamide AVP (DGAVP) 1 h prior to the retention test facilitated passive avoidance behavior (increase in avoidance latency). Conversely i.c.v administration of vasopressin antiserum resulted in attenuation of the avoidance behavior. The effects of altered passive avoidance behavior as a consequence of i.c.v. administration of DGAVP or vasopressin antiserum were studied on the primary antibody response. Again an inversely proportional relationship between the avoidance latency and the magnitude of the primary antibody response was observed. The results show that the immune system can specifically react to graded environmental stimuli. It is hypothesized that vasopressin may be an endogenous mediator determining the outcome of the avoidance behavior and the primary antibody response.