1. Because changes 5-HT(1A) receptor number do not occur following repeated agonist treatment, we hypothesized that the basis for 5-HT(1A) receptor desensitization involves changes in receptor-G protein coupling. We measured the effect of repeated agonist administration on 5-HT(1A) receptor-stimulated [(35)S]-GTPgammaS binding in forebrain areas, (i.e. anterior cingulate cortex, lateral septum, hippocampus, entorhinal cortex), and serotonergic cell body areas, the dorsal and median raphe nuclei. 2. Following treatment of rats with (+/-)8-OH-DPAT (1 mg kg(-1), s.c.) for 7 or 14 days, 5-HT(1A) receptor-stimulated [(35)S]-GTPgammaS binding was significantly attenuated in both the dorsal and median raphe nuclei. 3. 5-HT(1A) receptor-stimulated [(35)S]-GTPgammaS binding was significantly attenuated in the CA(1) region of the hippocampus after 7, but not 14 days of 8-OH-DPAT administration. 5-HT(1A) receptor-stimulated [(35)S]-GTPgammaS binding was not altered in other forebrain areas examined. 4. The binding of [(3)H]-MPPF to 5-HT(1A) receptor sites was not altered in any brain region examined following repeated agonist administration, suggesting that the observed changes in (+/-)8-OH-DPAT-stimulated [(35)S]-GTPgammaS binding were not due to changes in 5-HT(1A) receptor number. 5. Our data indicate that in serotonergic cell body areas the regulation of presynaptic 5-HT(1A) receptor function following repeated agonist administration occurs at the level of receptor-G protein interaction. In forebrain areas, however, the regulation of postsynaptic 5-HT(1A) receptor sensitivity appears not to be at the level of receptor-G protein coupling.