Excitatory and oscillogenic action of psychostimulant sydnophen on identified serotoninergic and peptidergic neurons was shown in the Helix brain. Sydnophen induced marked depolarizing shift, increase of the firing activity, activated excitatory synaptic input and generation of endogenous slow depolarizing potentials and burst activity in these neurons. The effect of sydnophen was blocked by naloxone and was imitated by the agonists of opiate receptors morphine and met-enkephalin. Majority of the other studied identified and non-identified neurons in the Helix brain did not react to sydnophen. Selective naloxone-dependent effect of sydnophen and the possible role of endogenous opioid system in the action of psychotropic agents are discussed.