The effect of several opiate receptor agonists on the responses of spinal cord neurons to putative inhibitory and excitatory amino acids was studied using an in vitro model system, cultured fetal mouse spinal cord neurons, and bath application of opiates. Intracellular recordings were made from the cultured neurons with conventional voltage recording techniques or under voltage clamp conditions. The putative amino acid neurotransmitters were applied by iontophoresis or micropressure ejection. Our main finding is that the opiate agonists, morphine and levorphanol (5-100 microM), consistently depressed the responses evoked by the putative inhibitory amino acid neurotransmitters glycine and beta-alanine but not GABA. Dextrorphan, the inactive isomer of levorphanol, also depressed the glycine and beta-alanine responses, but higher concentrations were required. The excitatory glutamate response was unaltered by these opiates. Leucine enkephalin, an opioid peptide, had no effect on the amino acid responses in the neurons where it was also tested. The opiate antagonist naloxone (10-100 microM) did not reverse the morphine or levorphanol depressions of the amino acid responses making it unlikely that opiate receptors mediate this effect. Strychnine was considerably more effective than morphine as a glycine antagonist, producing depressions at nM concentrations compared to the microM concentrations required for morphine. Preliminary studies indicate that both morphine and strychnine act in a non-competitive manner. However, additional studies will be required before the sites of action for these agents can be identified. The possible pharmacological or toxicological significance of the present work remains to be determined. Considering the high doses of opiates (microM concentrations) required to depress the glycine and beta-alanine responses, it is unlikely that this action is relevant to normal therapeutic situations. However, such concentrations of opiates are often utilized in pharmacological studies and may be achieved when opiates are applied by iontophoresis. Our data indicate that consideration of the present opiate action should be made when microM concentrations or iontophoretic application of opiates are used for pharmacological studies of CNS tissue.