Thyroid hormones (THs) are well known for their genomic effects but recently attention has focused also on their nongenomic actions as rapid modulators of membrane receptors. Here we show that thyroxine (T4) and 3,3',5'-l-triiodothyronine (T3) rapidly decrease N-methyl-d-aspartate (NMDA)-evoked currents in rat hippocampal cultures with potency in the micromolar range. The effect is not mediated by glutamate or glycine binding sites as an increase in agonist or glycine concentration does not alter TH potencies. Furthermore THs' effect on NMDA receptors is independent of voltage and of subunit composition. The mechanism of THs' antagonistic effect does not involve PKC phosphorylation of NMDA receptors since neither blocking nor stimulating PKC changed THs' modulation. T3, but not T4, inhibits also kainate-evoked currents in hippocampal neurons in culture. In hippocampal pyramidal neurons in slice, T3, but not T4, significantly reduced the frequency of miniature excitatory postsynaptic currents (mEPSCs) without affecting their amplitude and decay. In cultured rat cortical neurons THs prevented glutamate-induced neuronal death at concentrations similar to those effective on glutamatergic receptors. Taken together our data show for the first time that THs can rapidly affect ionotropic glutamatergic receptors in hippocampal neurons, an effect that could have an important role in their modulation of brain function in physiological and pathological states.