The possibility that glucocorticoids regulate specific guanine nucleotide binding regulatory proteins (G proteins) was investigated in rat cerebral cortex. Corticosterone was administered to normal and bilaterally adrenalectomized rats, and hormone regulation of individual G-protein subunits was investigated in cerebral cortex in three ways: (i) immunoblot analysis of subunit protein, (ii) hybridization blot analysis of subunit mRNA, and (iii) ADP-ribosylation analysis of stimulatory G protein (Gs alpha) subunits. Chronic (7 days) corticosterone administration to normal rats increased levels of Gs alpha immunoreactivity, mRNA, and ADP-ribosylation but decreased levels of inhibitory G protein (Gi alpha) mRNA and tended to decrease levels of Gi alpha immunoreactivity. In contrast, levels of Go alpha and G beta immunoreactivity and mRNA were not influenced by corticosterone treatment. In adrenalectomized rats, corticosterone treatment produced a 25-50% increase in the levels of Gs alpha immunoreactivity, mRNA, and ADP-ribosylation, whereas the hormone produced a 20-35% decrease in the levels of Gi alpha immunoreactivity and mRNA. Adrenalectomy, without corticosterone replacement, produced the opposite effects on Gs alpha and Gi alpha compared to sham-operated controls, indicating that these G proteins are regulated by this class of steroid hormone under physiological conditions in vivo. The results indicate that specific G-protein subunits--namely, Gs alpha and Gi alpha--are under the coordinated control of glucocorticoids in rat brain and demonstrate that G proteins are physiological targets of glucocorticoids in vivo. Possible roles played by these G-protein responses in mediating the effects of glucocorticoids on brain function are discussed.