Whole-cell patch clamp recordings from isolated spinal cord neurons from the frog Rana temporaria were made to study the interaction of processes induced by application of GABA and glycine. The amplitudes of currents evoked by application of glycine did not change with time, while the amplitudes of GABA-mediated currents decreased two-fold during the first 15 min of the experiment and stabilized at the new level. Neuron responses to simultaneous application of GABA and glycine were always smaller than the sum of the responses to separate application of these neurotransmitters. On application of GABA and glycine at the same concentration (5 mM), the amplitude of the response to simultaneous application decreased with time, reaching the level of the glycine-mediated response. A mixture of glycine and GABA at 8 microM and 5 mM, respectively, gave settled responses which were larger than the largest individual response by more than obtained with other mixtures. These data provide evidence that frog motoneurons may express receptors activated by both GABA and glycine.