['Angiotensin II (Ang II), a potent vasoconstrictor, affects the growth and development of hematopoietic cells. Mixed findings have been reported for the effects of angiotensin-converting enzyme (ACE) inhibitors on radiation-induced injury to the hematopoietic system. We investigated the consequences of different regimens of the ACE inhibitor captopril on radiation-induced hematopoietic injury.', 'C57BL/6 mice were either sham-irradiated or exposed to (60)Co total body irradiation (0.6 Gy/min). Captopril was provided in the water for different time periods relative to irradiation.', 'In untreated mice, the survival rate from 7.5 Gy was 50% at 30 days postirradiation. Captopril treatment for 7 days prior to irradiation resulted in radiosensitization with 100% lethality and a rapid decline in mature blood cells. In contrast, captopril treatment beginning 1 hour postirradiation and continuing for 30 days resulted in 100% survival, with improved recovery of mature blood cells and multilineage hematopoietic progenitors. In nonirradiated control mice, captopril biphasically modulated Lin(-) marrow progenitor cell cycling. After 2 days, captopril suppressed G(0)-G(1) transition and a greater number of cells entered a quiescent state. However, after 7 days of captopril treatment Lin(-) progenitor cell cycling increased compared to untreated control mice.'] Does timing of captopril administration determine radiation protection or radiation sensitization in a murine model of total body irradiation?