The β adrenoceptor antagonist radioligand [3H] dihydroalprenolol (DHA) has been used to characterise β adrenoceptors in membranes prepared from rat renal glomeruli. Association of the ligand was rapid and had reached equilibrium within 10 mins at 37°C. Dissociation occurred in two distinct phases, a rapidly dissociating phase (low affinity site) and a slowly dissociating phase (high affinity site). The KD value for the high affinity site calculated from the kinetic experiments was 0.8 nM. Saturation analysis of binding gave comparable values for KD (1.77 nM) and demonstrated that membranes from glomeruli had four times the density of binding sites measured in renal cortex. In all saturation studies Hill coefficients were not significantly different from unity. Binding was stereoselective with respect to the (-) isomers of isoprenaline and propranolol and the potency of the selective displacing agents betaxolol (β1 adrenoceptors) and ICI 118,551 (β2 adrenoceptors) indicated that the receptors are of the β1 subtype.