1. A method is described for the measurement of [des-Asp1]-angiotensin I (ANG I 1/2) in blood. 2. Exogenous ANG I 1/2 was rapidly metabolized in conscious sodium replete sheep. 3. Inhibition of angiotensin converting enzyme activity by SQ 14 225 (captopril) abolished the production of angiotensin III (ANG III) and the pressor response to infused ANG I 1/2. 4. The endogenous blood concentration of ANG I 1/2 was very low and not significantly elevated by sodium depletion. 5. Although in vivo pulmonary conversion of ANG I 1/2 to ANG III may occur, this pathway appears to be only of minor importance in sheep. 6. In the presence of converting enzyme inhibition by SQ 14 225, the arteriovenous ratio of ANG I 1/2 was 1.29 (s.e.m. = 0.25). This indicates pulmonary production of ANG I 1/2 and the presence in the lung of a peptidase which hydrolyses aspartic acid from angiotensin I.