Recent observations made in our laboratory have shown that metergoline is a selective 5-hydroxytryptamine (5-HT) antagonist in the cerebral cortex. Fluoxetine is a reportedly selective 5-HT neuronal uptake blocker. In the present investigation these drugs have been used to examine the existence of a putative 5-HT input to the cerebral cortex. In rats anaesthetized with a mixture of methoxyflurane, nitrous oxide, and oxygen, stimulation of the median raphé nucleus or of the ipsilateral cortical surface and iontophoretically applied 5-HT decreased the firing of the deep cerebral cortical neurones. Metergoline antagonized the 5-HT-induced depression and reduced the duration of inhibition produced by raphé nucleus stimulation. The inhibition produced by cortical surface stimulation was unaffected. Fluoxetine potentiated and prolonged the inhibition of cortical neurones induced by either 5-HT application or by raphé nucleus stimulation. These results suggest that the cerebral cortical neurones are inhibited by an ascending 5-HT-containing pathway.