Rat urine was found to contain a component showing cross-reactivity with antibody against rat plasma angiotensinogen. Sodium dodecyl sulfate polyacrylamide gel electrophoresis of rat urine revealed antigenic bands corresponding to the molecular weights of plasma angiotensinogen. The urinary angiotensinogen excretion in 8 rats, determined by direct radioimmunoassay, was 2.70 +/- 0.21 micrograms/day. Induction of acute inflammation in rats by injection of lipopolysaccharide caused about a 7-fold increase of urinary angiotensinogen excretion in the 24 hr after injection, with a concomitant elevation of plasma angiotensinogen. Neither sodium depletion nor loading by a low- or high-sodium diet altered the urinary excretion of angiotensinogen. These results suggest that the angiotensinogen present in rat urine is derived from that in plasma, although the level of excretion is too low to have any influence on the plasma level of angiotensinogen.