L-[1-13C] phenylalanine breath tests (PheBTs) have been used to determine the hepatocyte functional capacity of patients. This study investigated the relationship between the PheBT parameter 13C excretion rate constant (PheBT-k) and activity of the phenylalanine metabolic rate-limiting enzyme phenylalanine hydroxylase (PAH) in rat liver. We noted that the time-course curves of 13C excretion presented as a single peak, which appeared 2 min after administration of L-[1-13C] phenylalanine (13C-Phe). 13C excretion during exhalation can be divided into a slow phase and a rapid phase. The PheBT-k in rats with carbon tetrachloride acute liver injury was.significantly lower than that of control rats. The rapid phase 13C disposition constants of the acute liver injured rats did not differ from that of the controls. The peak value of 13C abundance in the breath of the acute liver injured rats was markedly higher than that of the control group. Total liver PAH activity in the acute liver injured rats was significantly lower than that in the control group. PheBT-k was highly correlated with the total activity of liver PAH (r = 0.92, P < 0.001). The present findings indicate that PheBT results reflect PAH activity levels. The PheBT-k parameter is a sensitive index that can be used to evaluate PAH function in the liver. In addition we demonstrated that the rodent model used in this study is a valuable tool for basic research studies of the breath test.