Harmaline (28 mumoles/kg u.v.), cold exposure (4C) or isoniazid (2.2 mmoles/kg s.c.) increased the cGMP content in rat cerebellar cortex several fold. Isoniazid but not harmaline or cold exposure increased cGMP in the deep cerebellar nuclei (nuclei interpositus, vestibularis and fastigius) and striatum. In rats treated with the nicotinamide antagonist 3-acetylpyridine (3-AP) (0.66 mumoles/kg i.p. 4 days before) the tremorogenic effect of harmaline and the increase of cerebellar cortex cGMP produced by this alkaloid was abated. Similarly the increase of cGMP following exposure to cold was reduced. In contrast isoniazid and glutamate (10 mumoles intraventricularly) increased cGMP to the same extent in control and 3-AP treated rats. Since 3-AP produces in rat a massive degeneration of the inferior olivary nucleus and of the climbing fibers but leaves intact all the other cerebellar elements, these experiments suggest that an increase of cGMP content in postsynaptic cerebellar elements (presumable Purkinje cells) may be an expression of an increased release of an excitatory transmitter from either the climbing fivers or the parallel fibers.