The effect of local X irradiation on cardiac α and β receptors was studied in Wistar rats. Animals were given local heart irradiation with single doses of 15 or 20 Gy and were examined after a range of latency times of 7 to 400 days. Using the radioactive ligands [3H]CGP-12177 and [3H]prazosin, the maximal binding capacity was determined from saturation experiments. At 7 days after 20 Gy the maximal binding capacity of both α and β receptors was reduced to below the level of untreated control animals. Subsequently it rose continually to a maximum of 160% of the control level for β receptors and 130% for α receptors at 400 days postirradiation. The antagonists affinity as judged from the dissociation constant for [3H]CGP-12177 and [3H]-prazosin did not change significantly. A similar effect was observed after 15 Gy. An increase in adrenergic receptors may represent an important pathogenetic link between early morphological and late functional changes in the pathogenesis of radiation-induced heart disease.