1. The olfactory bulb contains high concentrations of zinc and copper. Whole cell recording techniques were used to examine the modulatory actions of zinc and copper on N-methyl-D-aspartate (NMDA), gamma-aminobutyric acid (GABA), and glycine receptors on rat olfactory bulb neurons in culture and acutely isolated from adult animals. 2. Zinc and copper were effective antagonists of both NMDA-and GABA-mediated currents. The median inhibiting concentrations (IC50s) for zinc were 19 microM for NMDA receptors and 17 microM for GABA receptors. The IC50s for copper were 22 microM for NMDA receptors and 18 microM for GABA receptors. 3. Zinc and copper (100 microM) had no effect on the steady-state, desensitized component of currents evoked by high concentrations of glycine (300 microM). In contrast, when low, nondesensitizing concentrations of glycine (30 microM) were used, 100 microM zinc dramatically potentiated the current and 100 microM copper blocked the current. 4. The effects of zinc and copper on NMDA-, GABA-, or glycine-mediated currents were not voltage dependent, irrespective of whether the effect was potentiation or inhibition. 5. These results provide the first evidence for an inhibitory effect of copper on NMDA receptors, and the first evidence that the effects of zinc and copper on glycine receptors are dependent on the state of the receptor. These results suggest that endogenous zinc and copper may act as allosteric neuromodulators of amino acid receptors on olfactory bulb neurons. Furthermore, zinc and copper may provide a mechanism for differential modulation of inhibitory transmission because of their distinct effects on glycine versus GABA receptors.