The large diurnal rhythm of circulating glucocorticoid levels was used to determine if physiological fluctuations of glucocorticoids were capable of modulating kainate-induced immediate early gene (IEG) activation, measured as AP-1 DNA binding activity, in rat brain since administered dexamethasone previously had been shown to be inhibitory. AP-1 activity in the cerebral cortex 1.5 h after kainate treatment measured at 08.00 h (4.9-fold control) was more than twice the stimulation obtained at 16.00 h (1.8-fold). These times of day are associated with reported low and high levels of circulating glucocorticoids at 08.00 and 16.00 h, respectively. To test if there was a causal relationship, kainate-induced AP-1 activity was measured at both times in adrenalectomized rats. Adrenalectomy abolished the attenuation of the response to kainate found in intact rats at 16.00 h, indicating that the diurnal fluctuations in circulating glucocorticoids contribute to modulation of IEG responses to kainate. Neither AP-1 activity in the hippocampus nor cyclic AMP response element activation in either brain region measured after kainate treatment was influenced by the time of day or by adrenalectomy. Immunoprecipitation of glucocorticoid receptors from cortical nuclear extracts co-precipitated c-Jun, indicating that the mechanism accounting for the supppression of AP-1 activity by glucocorticoids may involve direct interactions between activated glucocorticoid receptors and AP-1 constituent proteins. These results extend previous reports that administered glucocorticoids inhibit AP-1 activity by demonstrating that this occurs with endogenous glucocorticoids as a consequence of the circadian rhythm of circulating glucocorticoids and demonstrate that responses to kainate vary dependent upon the time of day. © 1995.