We have found that paraquat (PQ), a widely used herbicide, causes wet dog shakes (WDS), which involve the central opioid system, in rats. A non-selective nitric oxide (NO) synthase (NOS) inhibitor, N(omega)-nitro-L-arginine (L-NA), but not its less active enantiomer, N(omega)-nitro-D-arginine, decreased the PQ-induced WDS in a dose-related manner. A selective neuronal NOS inhibitor in vivo, 7-nitroindazole, also decreased the PQ-induced WDS. Although an opioid receptor antagonist, naloxone, reversed the suppressive effect of these NOS inhibitors on the PQ-induced WDS, L-arginine, an NO precursor, had no effect on it. These findings suggest that the suppression of the PQ-induced WDS by NOS inhibition is associated with the central opioid system and is insusceptible to exogenous L-arginine.