Caerulein caused a marked decrease in levels of guanosine 3',5'-cyclic monophosphate (cGMP) in the cerebellum in rats. This effect was observed to be dose-dependent after the intraperitoneal administration of caerulein for doses over 20 micrograms/kg and lasted for about 4 hr in doses of 100 micrograms/kg. However, in vagotomized rats, caerulein failed to alter the level of cGMP in the cerebellum. Caerulein suppressed harmaline-induced increases in cGMP in the cerebellum for more than 30 hr. In contrast, the increases in levels of cGMP in the cerebellum, induced by treatment with methamphetamine, apomorphine and picrotoxin, were not inhibited by pretreatment with caerulein. These results suggest that the peripheral administration of caerulein can inhibit the activity of climbing fibers for a long period of time in the cerebellum of the rat through the stimulation of the abdominal vagus nerves.