Pig brain cortex synaptosomes and slices preincubated with 3H-5-hydroxytryptamine (3H-5-HT) were superfused with physiological salt solution containing citalopram (an inhibitor of 5-HT uptake), and the effects of indolethylamines and 5-HT receptor antagonists on the potassium- or electrically evoked 3H overflow were determined. The potassium (25 mmol/l)-evoked tritium overflow from cortex synaptosomes was inhibited by 5-HT; the inhibitory effect of 5-HT was counteracted by metitepine, which, by itself, did not affect the evoked overflow. 5-Methoxytryptamine (examined in the absence of citalopram) also produced an inhibition of the evoked overflow. In cortex slices, the electrically (3 Hz) evoked overflow was inhibited by 5-HT and 5-carboxamidotryptamine. The inhibitory effect of 5-HT was antagonized by metitepine, which, given alone, increased the evoked overflow, but was not attenuated by ketanserin and ICS 205-930 ([3 alpha-tropanyl]-1H-indole-3-carboxylic acid ester), which, by themselves, did not influence the evoked overflow. The present results suggest that the serotoninergic nerve fibres of the pig brain cortex are endowed with presynaptic 5-HT1 receptors, which can be activated by endogenous and exogenous 5-HT.