Abstract. The pharmacological properties of the presynaptic 2-adrenoceptors modulating the release of serotonin in rat and rabbit brain cortex (2-heteroreceptors) were compared with the properties of presynaptic 2-autoreceptors in the same brain area. Brain cortex slices were preincubated with [3H]-serotonin or [3H]-noradrenaline and then superfused and stimulated by brief high-frequency pulse trains. The 2-adrenoceptor agonist bromoxidine reduced the electrically evoked overflow of tritium in experiments with both [3H]-noradrenaline and [3H]-serotonin and in brain slices from either species. The antagonists phentolamine, idazoxan, (+)-mianserin, rauwolscine, 5-chloro-4-(1-butyl-1,2,5,6-tetrahydropyridin-3-yl)-thiazole-2-amine (ORG 20350), 2-(2,6-dimethoxyphenoxyethyl)aminomethyl-1,4-benzodioxane (WB 4101), ()-mianserin and corynanthine caused parallel shifts of the concentration-inhibition curves of bromoxidine to the right. Negative logarithms of antagonist dissociation constants pKd were calculated from the shifts. In the rat, the 2-autoreceptor pKd value of each single antagonist was similar to its 2-heteroreceptor pKd value, maximal difference 0.4, giving a close correlation, r=0.97 (P0.001). In the rabbit equally, the 2-autoreceptor pKd value of each single antagonist was similar to its 2-heteroreceptor pKd value, maximal difference 0.4, again yielding a close correlation, r=0.96 (P0.001). However, antagonist pKd values at rat 2-autoreceptors differed from those at rabbit 2-autoreceptors, r=0.70 (P0.05), and antagonist pKd values at rat 2-heteroreceptors differed from those at rabbit 2-heteroreceptors, r=0.64 (P0.05). Comparison with radioligand binding experiments from the literature indicated that, in the rat, both auto- and heteroreceptors conformed best to the 2D subtype (r0.97, P0.01 for pKd correlation with binding sites in rat submaxillary gland) whereas, in the rabbit, they conformed best to the 2A subtype (r0.93, P0.01 for pKd correlation with binding sites in HT29 cells). It is concluded that, in both the rat and the rabbit, the 2-adrenoceptors modulating the release of serotonin are pharmacologically identical with the presynaptic 2-autoreceptors. However, rat 2-autoreceptors and -heteroreceptors differ pharmacologically from rabbit 2-autoreceptors and -heteroreceptors. Presynaptic 2-auto- as well as -heteroreceptors are 2D in the rat and 2A in the rabbit.