β-Adrenergic receptor subtypes, β1 and β2, were studied during pre-and postnatal development in the rat brain. [125I]Iodocyanopindolol (6–300 pmol/L) binding assays in the presence of 5-hydroxytryptamine (0.6–6 μmol/L) were used to measure exclusively β-adrenergic receptors. In forebrain tissue, saturable and stereoselective binding was detected on gestational day 13. The amount of β-adrenergic binding increased until postnatal day 23, when adult values were reached. The dissociation constants of [125I]iodocyanopindolol binding remained the same throughout development, as did the affinity of several β-adrenergic and non-β-adrenergic compounds. The proportion of the β2-adrenergic receptors was determined using the β1-selective antagonist ICI-89406 (7–150 nmol/L) and was found to change from 65% in prenatal forebrain tissue to 28% in adulthood. In cerebellum/medulla pons tissue, however, the proportion of β2-receptor binding (80%) remained unchanged during the whole developmental period.