Recent studies of circulating lipoproteins in man and the rat have shown that apolipoprotein B in plasma exists in two immunologically distinct forms with apparent Mr, in the rat, of approximately 335,000 (apoB335K) and 240,000 (apoB240K). With isolated, perfused organs and single and double isotope labeling methods in vivo (1979) J. Biol. Chem. 254, 7316), we have now found that the rat liver produces both apoB variants in abundance while the small intestine produces apoB240K but only trace amounts of apoB335K. The hepatic apoB335K/apoB240Kleucine incorporation ratio ranged from 0.5-1.7 under different conditions, suggesting physiological regulation of the relative amounts produced. Disappearance of both newly synthesized hepatic apoB variants from plasma followed first order kinetics. The circulating half-life of hepatic apoB335K was 2.4 times that of hepatic apoB240K, indicating independent catabolism of the two variants and consistent with the hepatic production of multiple apoB-containing lipoproteins with different metabolic properties. The half-life of apoB240K from the intestine was even shorter than that of apoB240K from the liver.