The present study investigates the role of nitric oxide (NO) on restraint stress (RS)-induced modulation of humoral and cell-mediated immune responses in rats and mice. RS produced suppression of humoral immune response, i.e., anti-SRBC antibody titre (7.38+/-0.32 versus 4.13+/-0.30; mean+/-S.E.M., P<0.001). In case of cell-mediated immunity, in delayed type hypersensitivity (DTH) response the change in paw volume decreased from 0.069+/-0.003mm (mean+/-S.E.M.) in control non-stressed group to 0.038+/-0.002mm in the stressed group (P<0.001) while percentage leucocyte migration inhibition (% LMI) decreased from 39.7+/-1.95 in control non-stressed animals to 15.2+/-1.07 in animals subjected to stress (P<0.01). Pretreating the animals with an NO precursor, l-arginine (1000mgkg - 1 , i.p.) antagonized the effect of RS on humoral (anti-SRBC antibody titre 6.50+/-0.27 versus 4.13+/-0.30, P<0.001) and cell-mediated (DTH response 0.066+/-0.002mm versus 0.038+/-0.002mm, P<0.001; % LMI 41.5+/-1.46 versus 15.2+/-1.07, P<0.01) immune responses. Administration of 7-nitroindazole (7-NI, 50mgkg - 1 , i.p.), an inhibitor of neuronal NO synthase, alone further enhanced the immunosuppressive effect of RS (anti-SRBC antibody titre 2.75+/-0.25 versus 4.13+/-0.30, P<0.001; DTH response 0.019+/-0.002mm versus 0.038+/-0.002mm, P<0.001; % LMI 5.0+/-1.08 versus 15.2+/-1.07, P<0.01). However, when given before l-arginine treatment, 7-NI reversed the effect of the latter drug on stress-induced immunomodulation (anti-SRBC antibody titre 3.00+/-0.27 versus 6.5+/-0.27, P<0.001; DTH response 0.043+/-0.003mm versus 0.066+/-0.002mm, P<0.001; % LMI 12.0+/-0.93 versus 41.5+/-1.46, P<0.01). Unlike its effect on RS-induced immune responsiveness, l-arginine (250, 500, 1000mgkg - 1 ) when given for 5-7 days to naive non-stressed animals produced dose dependent suppression of both humoral (anti-SRBC antibody titre 6.4+/-0.32 versus 5.4+/-0.32, 4.0+/-0.27, 3.1+/-0.30, respectively) and cell-mediated (DTH 0.065+/-0.003mm versus 0.064+/-0.004mm, 0.039+/-0.003mm, 0.020+/-0.002mm, respectively and % LMI 37.52+/-1.58 versus 30.48+/-1.07, 28.18+/-1.22, 19.76+/-0.83, respectively) immune responses. 7-NI significantly blocked these immunosuppressive effects of l-arginine (anti-SRBC antibody titre 6.0+/-0.38 versus 3.1+/-0.030, P<0.001; DTH response 0.056+/-0.004mm versus 0.020+/-0.002mm, P<0.001; % LMI 34.76+/-1.31 versus 19.76+/-0.83, P<0.01). However, 7-NI when given to non-stressed animals failed to modulate immune responsiveness. Thus, NO appears to play an important role in RS-induced immunomodulation and these effects are different from its effect on immune responsiveness in non-stressed animals.