Bovine pulmonary arterial endothelial cells (BPAE) were exposed to a single dose 0, 5, 10, 20, or 30 Gy, in culture. Angiotensin converting enzyme (ACE) activity was determined in confluent monolayers, under first-order reaction conditions, at 6, 24, 48, and 96 hr after treatment, using [3H]benzoyl-Phe-Ala-Pro as substrate. Irradiation decreased the number of viable endothelial cells in a dose- and time-dependent manner, beginning at 24 hr after 5 Gy and reaching a maximum effect (21% survival) at 96 hr after 30 Gy. Total amount of protein per monolayer decreased during the same time intervals, whereas protein content per cell rose, signifying a radiation-induced hypertrophy of viable cells. When expressed per million surviving cells, ACE activity increased in a time- and dose-dependent manner, beginning at 24 hr after 5 Gy and reaching a maximum fourfold increase at 96 hr after 30 Gy. However, when expressed per culture well, ACE activity decreased in a time- and radiation-dependent manner. These results suggest that although at the lowest radiation dose (5 Gy), the increase in ACE activity per cell compensated for the enzymatic activity lost due to extensive cell death, at higher doses (10, 20, and 30 Gy), the increase in ACE activity per cell could not keep up with the decrease in the number of viable endothelial cells, leading to an overall decrease in ACE activity per culture well.