The development of beta 1- and beta 2-adrenergic receptors was studied in rat cerebral cortex and cerebellum. In the cerebral cortex, which contains mostly beta 1-adrenergic receptors, total beta-adrenergic receptor density increased sharply between postnatal days 10 and 21. The density of receptors remained fairly constant through 6 weeks of age and then subsequently declined. The proportion of beta 1 and beta 2 receptors was relatively constant throughout the development of the cerebral cortex. The development of the two receptor subtypes thus paralleled the development of total beta-adrenergic receptors in the cerebral cortex. The ontogeny of beta-adrenergic receptors in the cerebellum, which contains mainly beta 2 receptors, was strikingly different from that observed in the cortex. Total cerebellar beta receptor density exhibited a slow but steady increase from postnatal day 5 through day 42. The density of receptors then plateaued and remained constant until the animals were approximately 6 months of age. Unlike the results obtained in the cortex, the relative proportions of beta 1 and beta 2 receptors in the cerebellum changed markedly during development. Between postnatal days 8 and 13 approximately 18% of the receptors were of the beta 1 subtype. This proportion steadily decreased with age, and in 3- and 6-month-old animals only approximately 2% of the receptors were of the beta 1 subtype. The results demonstrate that the two subtypes of beta-adrenergic receptors can have different developmental patterns in the same brain area, and that a single receptor subtype can follow different developmental patterns in different brain regions. Possible correlations between the ontogeny of beta 1 and beta 2 receptors and various developmental events are discussed.