We have developed a procedure to measure the rates of apolipoprotein (apoB) and triglyceride secretion from the liver of an individual mouse. Using the well-characterized method of Triton WR-1339 injection to block peripheral removal of newly secreted VLDL, the rate of triglyceride accumulation is monitored and at the end of the experimental period, blood is extracted for quantitative VLDL preparation. ApoB species in isolated VLDL are analyzed by sodium dodecyl sulfate-polyacrylamide gel electrophoresis, and the mass of the apoB-48 and apoB-100 species are estimated by Coomassie staining and laser densitometric scanning, using known quantities of LDL-apoB-100 as standards. This methodology was applied to measure the rate of apoB secretion in male and female FVB/N mice and we found that the molar ratio of newly secreted ApoB-48 to B-100 is 4.6 in the male, and 3.8 in the female. Measurements of the steady state apoB levels indicate that liver-derived apoB-48 is cleared from the circulation 7.1 times faster than B-100 in the male and 4.7 times faster in the female mouse. VLDL apoB-48 fractional turnover is approximately 1800 pools per day in both the male and female mouse (1814 +/- 139 vs. 1831 +/- 365 respectively, P = 0.92). ApoB-100 fractional turnover rates are much slower and show a statistically significant difference between males and females (255 +/- 19 pools per day vs. 386 +/- 65 pools per day, respectively, P = 0.006). This procedure provides for quantification of secretory rates of these apo proteins in vivo, and may be useful for studying the effects of genetic manipulation on the simultaneous secretion of apoB-48- and apoB-100-containing VLDL, afforded by the panoply of transgenic mouse models now available for study, as well as for effects of diet and drug therapy.