Abstract In an attempt to elucidate molecular mechanisms related to low protein diet-induced fatty livers, apolipoprotein (apo) mRNA and its expression in the liver were studied in rats. Rats were fed for 14 days a diet containing 5% and 20% casein, and whey protein equivalent in protein to these caseins. In the protein-deficient groups, liver triacylglycerols elevated to 1.5–2 fold higher than did the diet sufficient groups. The mRNAs for apo A-IV, apo B and apo E were 20 to 50% lower in the deficient groups than in the sufficient groups, but apo A-I mRNA was not reduced. The rate of transcription of apo A-IV, B, and E genes in hepatic nuclei isolated from those animals was not altered with different levels of dietary protein. Apo B concentration in the serum was lowered and correlated positively with apo B mRNA abundance but the concentrations of apo IV and apo E were elevated in the deficient groups. It thus appears that a low protein diet decreases apo B synthesis through a posttranscriptional mechanism and thereby depresses secretion of triacylglycerol-rich lipoproteins from the liver.