As a first step toward understanding the regulation of apoprotein B (apoB) secretion in man, we have simultaneously studied the production of very-low-density lipoprotein (VLDL) apoB and VLDL triglycerides (TG) in six subjects with plasma TG levels of 153-344 mg/dl, during consumption of control and high-carbohydrate diets. 125I-labeled VLDL and [3H]glycerol were injected intravenously, and blood samples obtained over a 48-h period were used to isolated VLDL 125I-labeled apoB and VLDL 3H-labeled TG. The complex radioactivity curves obtained were analyzed by a multicompartmental model of VLDL metabolism. In accord with previous data, plasma VLDL TG levels were increased in five of six subjects (mean increase of 76% for the group), and in four of the six subjects the VLDL apoB concentrations were increased significantly during the high-carbohydrate period. The estimated rate of secretion of VLDL TG was higher in all six subjects, from 9.4 to 23.5 mg/kg-1 X h-1 on the control diet to 14.0-65.5 mg X kg-1 X h-1 on the carbohydrate diet (P < 0.001). In contrast, there was no increase in the rate of secretion of VLDL apoB in any of the subjects compared to their control period with values ranging from 0.60 to 1.59 mg X kg-1 X h-1 during the control period to 0.63-1.47 mg X kg-1 X h-1 after carbohydrate induction. These results indicate that physiological perturbations of VLDL TG production need not be accompanied by an alteration in VLDL apoB production.