Abstract 1. 1. In in vivo and in vitro studies in rats, the effects of dextromethorphan (DM), dextrorphan (DX), and levorphanol (LV) were compared with those induced by kappa and sigma opiate agonists. 2. 2. In rat hippocampal slices all the morphinans were able to pertubate the CA1 hippocampal synaptic transmission, while only DX and LV affected the N- methyl- d-aspartate excitability through a possible interaction at sigma opiate receptors. 3. 3. On the other hand EEG studies show that only DX appears to act as a full agonist at sigma opiate receptors. 4. 4. Present data demonstrate diversified electrophysiological properties of morphinans both in in vitro and in vivo studies.