The possible antinociceptive effect of a Rho-kinase inhibitor, (+)-(R)-trans-4-(1-aminoethyl)-N-(4-pyridyl) cyclohexanecarboxamide dihydrochloride monohydrate (Y-27632), was investigated in mice by using the hot-plate and abdominal constriction response (writhing) tests. In addition, the expression of Rho-kinase protein (ROCK-2) was studied in the mouse brain and spinal cord by Western blotting. Male balb/c mice (n=8, for each group) were used in the experiment. Hot-plate latency and the number of writhes were recorded in control and in Y-27632-treated (1-5 mg/kg, i.p.) groups. Y-27632 (1 mg/kg) did not affect hot-plate latency; however, it considerably diminished the number of writhes, from 89+/-12 in control to 30+/-6 in the mice treated with 1 mg/kg Y-27632 (P=0.001). At a higher dose (5 mg/kg), Y-27632 prolonged the hot-plate latency from 8.7+/-1.0 s to 14.4+/-1.7 s (P=0.005) and decreased the number of writhes from 80+/-8 to 24+/-7 (P=0.002). Western blot analysis revealed that mouse spinal cord and brain homogenates expressed ROCK-2 protein. These results indicate that Rho-kinase may be involved in nociception and that its inhibitors, such as Y-27632, may represent a new type of antinociceptive drug.