This study investigates the effects of intrathecally administered FK888, a novel dipeptide NK1 receptor antagonist, on the formalin test in the rat. Subcutaneous formalin induces biphasic episodes of flinching behavior at 0-5 min (Phase 1) and 10-60 min (Phase 2) after treatment. FK888 administered intrathecally 10 min before the formalin injection depresses the Phase 2 flinching behavior in a dose-dependent manner, but not the Phase 1 flinching behavior. FK888 has no effect on the Phase 2 flinching behavior when administered intrathecally 60 min before or 9 min after the formalin injection. Intrathecally administered FK888 has no effect on the motor function. These results suggest that FK888 is good drug to study the role of the NK1 receptor in the spinal cord of the rat.