The effect of Cd2+, as one of the most widespread toxic environmental pollutants, was studied on gamma-aminobutyric acid (GABA) evoked responses of identified neurons in the central nervous system of the pond snail, LYmnaea stagnalis L. (Gastropoda). In the experiments, the modulation of the action of GABA both on neuronal activity (current clamp recording) and on the a GABA activated membrane Cl- current (voltage clamp studies) has been shown. It was found that: 1. GABA could evoked three different various types of response in GABA sensitive neurons: i) hyperpolarization with strong inhibition of ongoing spike activity, ii) short depolarization with an increase of spike the activity, iii) biphasic respone with a short excitation followed by a more prolonged long inhibition. 2. In low-Cl- solution the inhibitory action of GABA was reduced or eliminated, but the excitatory one was not or only moderately affected. 3. CdCl2 inhibited the GABA evoked hyperpolarization, but left intact or only slightly reduced the excitation evoked by GABA. 4. The inward Cl- current evoked by GABA at a -75 mV holding potential was slightly augmented in the presence of I micromol/l Cd2+, but was reduced or blocked at higher cadmium concentrations. The effect of Cd2+ was concentration and time dependent. 5. Parallel with reducing the GABA evoked current, cadmium increased both the time to peak and the half inactivation time of the current. 6. CdCl2 alone, in 50 micromol/l concentration, induced a 1-2 nA inward current. The blocking effect of cadmium on GABA activated inhibitory processes can be an important component of the neuro-toxic effects of this heavy metal ion.