Using a push-pull cannula technique and an isotopic method for estimating [3H]serotonin continuously synthesized from [3H]tryptophan, the effects of changes in the release of serotonin in the dorsalis raphe nucleus on in vivo release of [3H]serotonin in the cat caudate nucleus were investigated. The increase in the release of serotonin in the dorsalis raphe nucleus caused by local application of parachlorophenylethylamine (10(-6) M) reduced striatal [3H]serotonin release. This inhibition in serotonin release in the striatum was blocked by the prior and continuous local superfusion of the dorsal raphe with methiothepin (10(-6) M), a serotonin autoreceptor antagonist. GABA (5 x 10(-5) M) applied to the dorsalis raphe reduced both local and striatal release of [3H]serotonin. However, picrotoxin (10(-5) M), a GABA A receptor antagonist applied locally in the dorsalis raphe nucleus increased [3H]serotonin release while decreasing striatal [3H]serotonin release. This decrease in serotonin release in the striatum was again blocked by continuous superfusion of the raphe with methiothepin. Furthermore, superfusion of serotonergic cell bodies of the dorsalis raphe nucleus with methiothepin alone never altered local release or striatal release of [3H]serotonin. These data strongly suggest that the release of serotonin from the cell body in the dorsalis raphe nucleus phasically controls release of the amine at the axonal nerve ending through serotonergic autoreceptors located on serotonergic nerve cell bodies in the dorsalis raphe nucleus. The origin of the serotonin released in the dorsalis raphe nucleus and the possibility that this type of regulation could be related to changes in nerve impulse conduction of the serotonergic raphe-striatal system are discussed.