We have previously shown that the sex steroid progesterone plays a modulatory role in amino acid physiology by suppressing excitatory responses of cerebellar Purkinje cells to glutamate and augmenting inhibitory responses of these neurons to GABA. In the present study using the rat, progesterone effects on neuronal responses to the specific excitatory amino acid agonists quisqualate, kainate and N-methyl-D-aspartate were tested using iontophoretic, extracellular single unit recording techniques. In addition, the effect of systemic administration of progesterone on quisqualate-evoked excitation was evaluated in the presence of the GABAA blocker bicuculline. Progesterone consistently attenuated excitatory neuronal responses to local application of all three excitatory amino acids by 40-51%, but exerted variable effects on combined administration of quisqualate and N-methyl-D-aspartate which were dependent on temporal and dose-related factors. Progesterone-induced attenuation of the quisqualate response was found to be mediated primarily by a non-N-methyl-D-aspartate receptor. In addition, bicuculline application did not block progesterone effects on quisqualate excitation, suggesting that the observed steroidal modulation of excitatory amino acid function is not secondary to progesterone-induced potentiation of GABA inhibition.