beta-Adrenergic receptors play an integral role in the modulation of cell function in the developing lung. In the rat, there are marked increases in beta receptor density in whole lung during postnatal maturation, but it is now known whether there are differential developmental changes in receptor density in specific cell types. Quantitative light microscopic autoradiography with [125I]iodocyanopindolol ([125I]ICYP) was used to determine maturational changes in beta-adrenergic receptor density in pulmonary arterial smooth muscle (ASM), bronchial smooth muscle (BSM), and alveolar lining cells (ALC) in rat lung during postnatal development (1 day to 6 mo). [125I]ICYP binding to whole lung sections revealed a single class of high-affinity receptors; agonist competitive binding studies suggested that the receptors are primarily of the beta 2 subtype. beta-Adrenergic receptor density in newborn (1 day) lung was lowest in ASM cells and was comparable in BSM cells and ALC. In contrast, in lungs from adult rats (3 mo), receptor density was similar in ASM versus BSM cells and was 2-fold greater in ALC. In addition, the maturational pattern of increasing receptor density differed in ASM compared with BSM and ALC. Receptor density in ASM increased 93% from 1 to 13 days, another 92% from 13 to 20 days, and was unchanged thereafter. In contrast, receptor density in BSM cells did not change from 1 to 13 days, but it increased 65% from 13 to 20 days, rose another 47% from 20 days to 3 mo, and increased an additional 24% from 3 to 6 mo.(ABSTRACT TRUNCATED AT 250 WORDS)