1. The specific binding of [3H]-muscimol, a gamma-aminobutyric acidA receptor (GABAA) agonist, to whole membranes of the guinea-pig urinary bladder was examined. In addition, the distribution of specific muscimol binding sites within the bladder was visualized by autoradiography. 2. It was demonstrated that in a frozen-thawed whole membrane preparation of the organ the specific binding of [3H]-muscimol is reproducible, reversible and saturable. 3. Saturable binding was of a single component with an equilibrium dissociation constant (Kd) of 12 nM and a maximal density (Bmax) of about 80 fmol mg-1 protein. 4. In displacement experiments with several model compounds, [3H]-muscimol binding sites showed the characteristics of a GABAA receptor site. 5. Autoradiographic experiments revealed uneven distribution of specifically bound [3H]-muscimol in the bladder. The density of binding sites was high in clusters within the smooth muscle layers of the bladder fundus and of the urethra, while the apex and the neck were not specifically labelled. 6. The present findings show that GABAA type receptor sites in the guinea-pig urinary bladder may be labelled by [3H]-muscimol in a specific and reproducible manner. Moreover, the localization of these binding sites is consistent with the presence of GABAA receptors in only a subpopulation of vesical ganglia.