We have previously reported that in primary rat hepatocytes, n-3 fatty acid (either eicosapentaenoic or docosahexaenoic) stimulates intracellular degradation of apoprotein B100 (apoB100) and apoB48 (Wang, H., Chen, X., and Fisher, E.A. (1993) J. Clin. Invest. 91, 1380-1389). There was a greater effect on apoB100 than on apoB48. Rat hepatoma (McArdle RH-7777) cell lines expressing carboxyl-truncated human apoB species apoB42, apoB28, and apoB18 were used to explore the relationship between the effects of n-3 fatty acids and apoB lipidation. After density gradient separation of conditioned media, apoB42 was found at d < 1.21 (lipid-rich), apoB18 at d > 1.21 (lipid-poor), and apoB28 in both fractions. After incubation with albumin complexed with eicosapentaenoic or docosahexaenoic acids, relative to oleic acid, the secretion of newly synthesized apoB was 35% (apoB42), 54% (apoB28), and 96% (apoB18). Further analysis of the apoB28 data showed decreased secretion of only lipid-rich (d < 1.21) apoB28. Pulse-chase studies indicated that with n-3 fatty acid there was increased intracellular degradation of apoB42 concomitant with its decreased secretion. We conclude that the ability of the n-3 fatty acids to promote apoB degradation correlated with the degree of lipidation of the secreted apoB, consistent with specialized intracellular pathways for the degradation of apoB and the assembly of buoyant, apoB-containing lipoproteins.