Abstract 5-HT 1A receptor function can be assessed in rat hippocampal and cortical membrane preparations as agonist-stimulated [ 35S]GTPγS binding. Membranes were preincubated in vitro with N-ethoxycarbonyl-2-ethoxy-1,2-dihydroquinoline (EEDQ). R(+)-8-hydroxy-2-(di- n-propylamino)tetralin [ R(+)-8-OH-DPAT]-stimulated [ 35S]GTPγS binding and [ 3H]8-OH-DPAT binding assays were used to assess 5-HT 1A receptor function and density, respectively. EEDQ decreased both R(+)-8-OH-DPAT-stimulated [ 35S]GTPγS and [ 3H]8-OH-DPAT binding in hippocampal and cortical membranes. The E max but not the EC 50 of R(+)-8-OH-DPAT to stimulate [ 35S]GTPγS binding was decreased by EEDQ in both preparations. Additionally, the IC 50 for EEDQ to reduce R(+)-8-OH-DPAT-stimulated [ 35S]GTPγS and [ 3H]8-OH-DPAT binding was the same for both brain regions in both assays. In contrast to EEDQ alone, agonist-stimulated [ 35S]GTPγS binding was not reduced in hippocampal membranes preincubated with EEDQ and the 5-HT 1A receptor antagonist N-[2-[4-(2-methoxyphenyl)-1-piperazinyl]ethyl]- N-2-pyridinyl-cyclohexanecarboxamide maleate (WAY 100,635), suggesting that EEDQ acts directly on the receptor. Due to parallel reductions in receptor density and maximal functional response, it is concluded that there is little or no reserve for 5-HT 1A receptor coupling to G α in these preparations. In addition, the sensitivity of hippocampal and cortical 5-HT 1A receptors to inactivation by EEDQ in vitro is the same.