Pharmacological studies using an in vitro model system were carried out to determine if naloxone, an opiate receptor antagonist, could have effects on neuronal membranes which were unrelated to its action as an opiate receptor antagonist. Intracellular recordings were made from cultured mammalian spinal cord neurons. Putative amino acid neurotransmitters and naloxone were applied by iontophoresis or superfusion. When naloxone was co-iontophoresed with the amino acids a depression of the GABA response resulted. This depression was dose-dependent and reversible. At the lower doses of naloxone tested, the depression was specific since the glycine and glutamate responses were unaffected. At the higher doses of naloxone tested, alterations in the glycine and/or glutamate responses and membrane input resistance were frequently observed. The naloxone depression of the GABA response did not appear to involve opiate receptors since (+)-naloxone, the inactive isomer, equally depressed the GABA response. Analysis of the effect of naloxone on GABA dose-response curves indicates that naloxone acts as a competitive antagonist at the neuronal GABA receptors. Similar results were obtained when naloxone was applied by superfusion. However, high concentrations of naloxone (0.1-1 mM) were required, suggesting that naloxone has a low affinity for the GABA receptor. These data indicate that under some experimental conditions naloxone could not be considered a specific opiate antagonist.