Release of 5-HT in the CNS is under the control of autoreceptors. These autoreceptors fall into two categories: cell body autoreceptors and terminal autoreceptors. The former inhibit 5-HT release through inhibition of cell firing; the latter through direct inhibition of release at the terminal. Cell body (or somatodendritic) autoreceptors belong to the 5-HT1A receptor subtype in all species studied so far. In the rat and mouse, the terminal autoreceptor is known to be a 5-HT1B receptor, whereas in human, pig, rabbit, and guinea pig, the terminal autoreceptor is thought to belong to the 5-HT1D receptor subtype. Until recently, the absence of a potent and selective 5-HT1D receptor antagonist has hindered this classification. We now present data with the novel 5-HT1D receptor antagonist, GR 127935, which demonstrates that in guinea pig cerebral cortex the terminal autoreceptor is a 5-HT1D receptor. In vitro [3H]5-HT release studies demonstrate that 5-HT inhibition of [3H]5-HT release is attenuated by GR 127935. In vivo, using the technique of microdialysis, GR 127935 and the non-selective antagonist methiothepin, when administered down the dialysis probe, potentiate extracellular levels of 5-HT. Both the in vitro and in vivo effects of these compounds are consistent with terminal autoreceptor blockade. However, when GR 127935 and methiothepin were administered systemically, both compounds inhibit extracellular levels of 5-HT. The most plausible explanations for this effect, such as partial agonism or activation of somatodendritic 5-HT1A receptors, are discussed.