Membrane currents evoked by glutamate were investigated in acutely dissociated neurons from the medial preoptic nucleus (MPN) of rat. Rapid application of glutamate induced a fast current component in all neurons studied. In addition, in > 50% of the neurons, a slow current component was elicited. The fast and the slow current components were selectively blocked by the AMPA-receptor antagonist NBQX and by the NMDA-receptor channel blocker MK-801, respectively. Rapid application of AMPA induced, in all neurons tested, currents with properties similar to the fast component of the glutamate-evoked currents whereas rapid application of NMDA induced, in approximately 75% of the neurons, currents similar to the slow component of the glutamate-evoked currents. The NMDA-evoked currents showed a marked outward rectification that was attributed to a potential-dependent block by extracellular Mg2+. The NMDA-evoked currents also required the presence of extracellular glycine in the micromolar range. In conclusion, the results show that MPN neurons respond to glutamate with currents that can be attributed to activation of ionotropic glutamate receptors of the AMPA-receptor type as well as of the NMDA-receptor type.