The present study was designed to investigate the effect of hypoxia on the pulmonary adrenoceptors in rats. The density and distribution of pulmonary adrenoceptors were studied by using radioligand binding techniques and autoradiography. The beta adrenoceptors in lung homogenates were found to decrease by 24 h of hypoxia, and to unexpectedly increase at 1 week of hypoxia. Exposure to hypoxia for 2 and 4 weeks led to another decrease in beta adrenoceptors. In contrast, the alpha 1 adrenoceptors were increased markedly at 24 h and 4 weeks of hypoxia. A somewhat higher density of alpha 1 adrenoceptors was also found at 1 week and 7 weeks of hypoxia. The affinity of beta and alpha 1 adrenoceptors did not change. Using a computer image analysis system, beta adrenoceptors were found to be present in higher density in bronchioles than in pulmonary blood vessels before and during hypoxia, while the reverse was true of alpha 1 adrenoceptors. Both beta and alpha 1 adrenoceptors were found in high densities in the parenchyma.