In the present study, the effects of PACAP27, PACAP38 and VIP in a concentration range from 10(-13) to 10(-6) M were studied in vitro on the spontaneous and directed mobility of lymphocytes from rat spleen and thymus. The results show that VIP and both PACAPs inhibit significantly and in a similar way the mobility of lymphocytes from thymus and spleen, and the maximal effects were observed at 10(-9) M and 10(-8) M. The three neuropeptides significantly increased cAMP concentrations. Moreover, incubation with increasing PMA concentrations showed a progressive enhancement of chemotaxis of lymphocytes, which was partially prevented by VIP, and both PACAPs. Incubation with forskolin caused decrease in the chemotaxis of thymocytes and splenocytes, and the presence of VIP or PACAP peptides was not synergistic in the inhibitory effect on lymphocyte chemotaxis, suggesting that the three neuropeptides and forskolin mediate their actions by the same intracellular pathway. This study showed the ability of the VIP receptor antagonist (N-Ac-Tyr1,D-Phe2)-GRF(1-29)-NH2 to partially reverse the inhibitory effect of both PACAPs and VIP on chemotaxis, suggesting that PACAP receptors are identical or very similar to VIP receptors in both thymocytes and splenocytes. These data suggest that PACAP27 and PACAP38 can be included as two novel immunoregulatory peptides that can modulate cell mobility on central and peripheral lymphoid organs.