Tail flick latency (TFL) was elevated following microinjection of both dibutyryl cyclic AMP (dbcAMP) was dibutyryl cyclic GMP (dbcGMP) at sites in the caudal reticular formation. TFL was also elevated following injection of dbcAMP but not dbcGMP into the periaqueductal gray. TFL was unaltered following administration of either agent into the lumbar intrathecal space. These actions are discussed with respect to the possible involvement of cyclic nucleotides in the analgesic action of morphine.