Carnitine and acetylcarnitine have been demonstrated to be present in the CNS and to be involved in cholinergic mechanisms, even if their exact role in neurotransmission is still unknown. This microiontophoretic study was carried out on single cholinoceptive neurons of the somatosensory cortex in the rat in order to analyze the effects of L- and D-carnitine and L-acetylcarnitine on the spontaneous firing and the neuronal responses to some putative transmitters. L-carnitine and L-acetylcarnitine increased the spontaneous discharge rate, while D-carnitine was found to be ineffective. L-acetylcarnitine clearly potentiated the cholinergic excitatory responses. On the contrary, L-carnitine was found to reduce cholinergic responses in a great percentage of units and to inhibit L-acetylcarnitine-induced excitatory responses. Atropine blocked the increase in firing rate induced by L-carnitine and L-acetylcarnitine, thus suggesting for both of them a muscarinic activity. No interactions were observed between carnitines and GABA and glutamate. These results show that carnitine and acetylcarnitine are stereospecific neuroactive compounds with a cholinomimetic activity. They may play a role in a modulatory system for the cholinoceptive cortical neuron.