Harmaline increases cerebellar 3':5'-cyclic guanosine monophosphate (cGMP) content in a dose-related manner; this increase is prevented by a pretreatment with 3-acetylpyridine (3-AP) (0.66 mmol/kg) which destroys climbing fibers and inhibits harmaline-induced tremor. The cerebellar cGMP content increases after isoniazid; this response remains unchanged in rats pretreated with 3-AP. Since isoniazid decreases cerebellar gamma-aminobuturic acid (GABA) levels, the increase in cGMP content might reflect a reduction in the availability of GABA at the level of postsynaptic receptors. Apomorphine (a dopamine receptor agonist) and haloperidol (a dopamine receptor blocker) increase or decrease the cGMP content of cerebellar cortex, respectively. Neither drug changes the guanylate cyclase activity of cerebellar homogenates; moreover their action on cerebellar cGMP content persists after 3-AP. Chloropromazine, like haloperidol, decreases the cerebellar cGMP content. The increase in cerebellar cGMP content elicited by apomorphine can be differentiated from that elicited by harmaline or isoniazid; presumably apomorphine indirectly activates mossy fibers. The decrease in cerebellar cGMP content elicited by haloperidol can be differentiated from that elicited by diazepam; perhaps haloperidol reduces the mossy fiber input to the cerebellum. We suggest that the cGMP content of cerebellar cortex fluctuates in response to changes in the afferent stimulatory input to the cerebellum; it increases when the activity of either climbing or mossy fibers is increased; it decreases when either of these two stimulatory inputs is reduced.