ABSTRACT.To localize fetal rabbit lung β-adrenoreceptors before and after glucocorticoid treatment, light microscopic autoradiography was performed with the reversible radiolabeled β-adrenergic antagonist, [3H]dihydroalprenolol, on day 26 of gestation. Autoradiograms of adult lung and fetal myocardium were also prepared. Examination of these autoradiograms showed densely labeled airways, alveoli, and myocardium. Specific labeling, defined as that prevented by incubation with 1-propranolol (1 μM), was 90%. Analysis of grain counts in the fetus showed that airways were more densely labeled than alveoli(p< 0.001), labeling was increased by treatment(p< 0.001) and treatment increased alveolar(p <0.002) but not airway labeling. Adult lungs were much more densely labeled than fetal, and fetal myocardial labeling was not altered by treatment. Adult untreated lung showed the same pattern as fetal untreated lung with airways being more densely labeled than alveoli (p< 0.001). To validate estimates of relative β-adrenoreceptor concentration derived from autoradiograms, comparisons to determinations of β-receptor concentraion from scintillaiton counting of lung section digests and from previously performed radiologand binding studies, This increaes is not present in either airway or myocardial β-adrenreceptors and is consistent with the known maturational effect of glucocortiocoid treatment on fetal olveolar function.