The effect of dietary protein concentrations on the hepatic expression of phenylalanine hydroxylase (PAH) and tyrosine aminotransferase (TAT) mRNA concentrations was studied in rats adapted to consume diets (18 or 50% casein) in a restricted schedule of 7 h (0900 to 1600) for 5 days. After 6 hours of feeding, TAT mRNA concentrations of rats adapted to 18% casein diet and fed acutely 6, 18 and 50% casein diet were 0.15, 0.84 and 5.08 fold respectively higher than mRNA concentrations of rats before feeding. After 17 hours of fasting, TAT mRNA concentrations of rats previously fed 6, 18 or 50% casein diet were -0.45, 1.76 and 9.11 fold respectively higher than mRNA concentrations of rats before they were fed. PAH mRNA concentrations showed a similar pattern. There was a -0.68, 1.63 and 2.5 fold rise of PAH mRNA concentrations in rats fed 6,18 and 50% casein diet during the feeding period, and -0.86, 2.32 and 9.33 fold rise after 17 hours of fasting. TAT and PAH mRNA concentrations of rats adapted to consume 50% casein diet and then changed to 6% or kept on the 50% casein diet showed a maximum peak 6 hours after the rats began to consume the diet; however, they decreased 5 hours after fasting. These results suggest that increasing concentrations of protein in the diet were able to increase the concentration of TAT and PAH mRNA, possibly in order to eliminate the excess of amino acids consumed. The concentration of TAT and PAH mRNA depended more on the protein content of the diet to which the rats were previously adapted.