We studied the effect of L-arginine on relaxation responses to sodium nitroprusside or SIN-1 (3-morpholinosydonimine-N-ethyl-carbamine) in the rat thoracic aorta incubated with endotoxin. Sodium nitroprusside or SIN-1 produced a reproducible relaxation in the aorta incubated for 12 h with endotoxin. However, the response to both nitrovasodilators was remarkably attenuated when the aorta was preincubated for 12 h with endotoxin and L-arginine. D-Arginine could not substitute for L-arginine. The attenuated response to sodium nitroprusside or SIN-1 was partially restored by the inhibition of nitric oxide (NO) production with N omega-nitro-L-arginine. Cycloheximide prevented the inhibitory effect of preincubation with L-arginine. These results suggest that the prolonged exposure to muscle-derived NO induces hyporesponsiveness to nitrovasodilators.