The possibility that the ACE inhibitors, enalapril and captopril, may decrease plasma EPO concentrations was studied in a single-blind, cross-over study in 10 healthy volunteers. Plasma EPO concentrations, haemoglobin concentration, red blood cell count, plasma creatinine concentration and mean arterial pressure were measured at baseline and after 28 days treatment with both ACE inhibitors. A significant fall in mean plasma EPO concentration occurred with both ACE inhibitors and returned to baseline after stopping the drugs. It is likely that ACE inhibitors decrease EPO formation, by inhibition of angiotensin-II production. This effect could be important in patients with renal failure, renal transplantation or other chronic conditions with an associated anaemia. Haematological parameters should be monitored in such patients when they are treated with an ACE inhibitor.