Simultaneous kinetics of apoproteins CII, CIII₁ and CIII₂ and VLDL-B were determined in six normal subjects initially while consuming a control diet and subsequently while consuming a high carbohydrate diet. Mean triglyceride levels in plasma and VLDL were significantly increased, from 110 to 173 mg/dl and 40 to 114 mg/dl respectively, by the high carbohydrate diet. Specific radioactivity of each C peptide was determined, following injection of ¹²⁵I-VLDL, in both VLDL and HDL from plasma samples taken over 48 hr using a technique involving analytical isoelectric focusing. Specific activity vs time curves for VLDL and HDL were almost superimposable on either diet, suggesting that CII, CIII₁ and CIII₂ were catabolized as a group. The high carbohydrate diet resulted in a significant increase in the mass of each C peptide which was brought about by an increase in production since no change in fractional removal rate (FRR) was observed. The pool size of VLDL apo B was also significantly increased and was a function of both increased production as well as decreased FRR. The proportion of CIII₂ mass relative to those of CII and CIII₁ was lower in both VLDL and HDL with the high carbohydrate diet. These observations indicate that the metabolism of the B and C apoproteins are independent of each other and do not respond in the same manner to changes in VLDL triglyceride and that increased triglyceride synthesis and secretion stimulates C apoprotein production.