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We have studied the possible involvement of nitric oxide (NO) in the contact hypersensitivity reaction. A biphasic response of ear swelling was observed at 2 h (early phase) and 24 h (late phase) after application of the antigen to picryl chloride (PC1)-sensitized CBA/J mice. Intravenous injection of NO synthase inhibitor, NG-nitro-L-arginine methyl ester (L-NAME), at the time of PC1 challenge, inhibited in a concentration-dependent fashion the antigen-induced contact hypersensitivity reaction. Low-dose (1 mg/kg) L-NAME inhibited the early-phase reaction but not the late-phase reaction. High-dose (250 mg/kg) L-NAME inhibited both early- and late-phase reactions. D-NAME (enantiomer of L-NAME) did not inhibit the antigen-induced ear swelling. High-dose (250 mg/kg) L-arginine increased both early and late phase reactions. D-Arginine (enantiomer of L-arginine) did no increase the antigen-induced ear swelling. L-NAME injection, however, did not suppress phenol-induce irritant inflammation. Treatment of mice undergoing PC1-induced contact hypersensitivity reaction with L-NAME reduced the production of interleukin-2 and interferon-gamma by draining lymph node cells. Treatment with L-arginine, on the other hand increased the production of interleukin-2 and interferon-gamma. These results suggest that NO plays a modulating role in contact hypersensitivity reaction.