An endogenous inhibitor of ?-aminobutyric acid (GABA) receptors was partially purified from bovine brain striatum. It was obtained as a low molecular weight fraction by gel filtration on Biogel P-2 and was adsorbed to Dowex AG 50W-X8, but not to Dowex AG 1-X8. It was ninhydrin-negative, basic, heat-stable substance. It caused dose-dependent inhibition of Na(+)-independent [(3)H]GABA bindings. Scatchard plot analysis of the [(3)H]GABA binding to GABA "B" receptor recognition site showed this inhibitor increased the K(d) value (24.1 nM to 3.6 nM) without changing the B(max). On the other hand, Scatchard plot analysis of the [(3)H]GABA binding to GABA "A" receptor recognition site showed that the inhibitor decreased number of binding sites (706 fmol/mg protein to 494 fmol/mg protein) without affecting the K(d) value. These results suggest that the endogenous inhibitor functions as a modulator for GABA(B) and GABA(A) receptors.