Evidence from an earlier study suggested that bombesin, neuromedin C and neuromedin B may play a role in spinal nociceptive transmission; iontophoretic administration of these peptides onto dorsal horn neurones in the cat was found to preferentially depress those neurones activated by noxious stimulation. Therefore, to further examine the possible function of these peptides in the spinal cord, the present study compares the effects of intrathecal administration of bombesin, neuromedin C and neuromedin B on reaction time in the tail flick test in the rat. Intrathecal injection of bombesin and neuromedin C to the lower lumbar vertebral level produced a dose-dependent decrease in reaction time which lasted up to 46 min. Similar administration of neuromedin B had a biphasic effect; there was a dose-dependent decrease in reaction time lasting about 6 min followed by a delayed increase in reaction time to above control values at 31-46 min. In addition, administration of these peptides induced behavioral responses such as spontaneous vocalization and vocalization in response to innocuous touch. These results provide physiological evidence for a role of neuromedin C and neuromedin B in sensory transmission at the spinal level. In this model, bombesin was a potent agonist which may selectively activate the neuromedin C receptor.