The interaction of heavy metals (HgCl2, CdCl2, CuCl2, PbCl2 and ZnCl2) and neurotransmitters (ACh, 5HT and DA) was studied on the excitable membrane of identified neurons of Lymnaea stagnalis and Helix pomatia. It was shown that, (1) The excitability and chemosensitivity of molluscan neurons were modified under the influence of the heavy metals Hg(2+), Cd(2+), Cu(2+), Pb(2+) and Zn(2+). (2) Change in excitability to transmitters occurred as a potentiation or depression of the evoked response both in duration of membrane polarization and in frequency of spike activity. (3) The chemosensitivity changes in various ways, namely: excitatory effect was totally eliminated; one component of the effect was depressed. Different neurons may show different reactions to the same heavy metal. (4) There were differences in the effects of various heavy metals. Hg(2+) has a more generalized effect than Cd(2+); Cu(2+), Pb(2+) and Zn(2+) were less effective in a number of neurons. The heavy metal effect was dose dependent, too. (5) Both inward and outward currents, which were evoked by neurotransmitters or voltage induced, were modified in most of the tested neurons. Both an increase and decrease of the membrane permeability occurred in different neurons in response to the same or different heavy metals. (6) The changes can be interpreted as a result of direct effect on specific ionic channels; modification of receptors binding ACh, 5HT, or DA; modification of intracellular processes responsible for the regulation of membrane permeability.