1. The effect of captopril on in vitro production of angiotensin I (ANG I), [Val5]-angiotensin II ([Val5]-ANG II) and [Val4]-angiotensin III ([Val5]-ANG-(2-8)) in central venous blood taken from sodium-deficient sheep was studied. 2. Captopril enhances in vitro production of ANG I but blocks the in vitro production of [Val5]-ANG II and [Val5]-ANG-(2-8). 3. The production of ANG I in blood is faster than that of [Val5]-ANG II and [Val5]-ANG-(2-8). 4. The half-life of [Val5]-ANG II and [Val5]-ANG-(2-8) in vitro in blood in the presence of captopril was 10 and 14 min, respectively. 5. This in vitro study suggests that the production of [Val5]-ANG II and [Val5]-ANG-(2-8) in blood forms a small part of the total body production of each peptide.