Lephetamine is a central analgesic, recently shown to be abused by drug addicts and to induce dependence in humans. The drug was applied microiontophoretically on single neurones of the somatosensory cortex of the rat in vivo. Its activity on the spontaneous and evoked firing rate was recorded. Morphine and naloxone were employed to verify the hypothesis that a mu-opiate mechanism of action could be involved. The most frequent response evoked by lephetamine was a dose-dependent excitation non-reversible by naloxone. On the other hand, units inhibited or apparently unaffected by the drug, showed a selective anti-glutamate (and partly anti-acetylcholine) effect, which was reversed by either systemically- or iontophoretically-administered naloxone. Long-lasting (8-12 min) applications of lephetamine caused a progressive desensitization of cortical neurones to the inhibitory and anti-glutamate effect. The inhibitory activity of lephetamine and morphine was additive and an increased neuronal excitability was shown by a post-inhibitory rebound of glutamate-induced neuronal activity. The action exerted by lephetamine on glutamate-induced excitations and on postsynaptic excitability, its reversibility by naloxone and the occurrence of acute tolerance allow the conclusion that only the inhibitory effect of lephetamine is mediated by an opioid mechanism. The lephetamine-induced excitations, not reversed by naloxone, are difficult to interpret as opioid-mediated.