To localize fetal rabbit lung beta-adrenoreceptors before and after glucocorticoid treatment, light microscopic autoradiography was performed with the reversible radiolabeled beta-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 l-propranolol (1 microM), was 90%. Analysis of grain counts in the fetus showed that airways were more densely labeled than alveoli (p less than 0.001), labeling was increased by treatment (p less than 0.001) and treatment increased alveolar (p less than 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 less than 0.001). To validate estimates of relative beta-adrenoreceptor concentration derived from autoradiograms, comparisons to determinations of beta-receptor concentration from scintillation counting of lung section digests and from previously performed radioligand binding studies, using membranes prepared from whole lung homogenates, were made. There is excellent agreement between estimates of relative receptor concentration and specific binding derived from the counting of autographic grains and both scintillation counting of lung section digests and previously performed radioligand binding of lung particulate. In all preparations, specific binding was (90%), increased with glucocorticoid treatment in fetal lung (50-100%), was greater in concentration in adult compared to fetal lung (7-10-fold), and did not increase in fetal myocardium with treatment.(ABSTRACT TRUNCATED AT 250 WORDS)