Abstract: Objective Using a small animal breath test model we designed and L-1-¹³C pheny-lalanine breath test (¹³C-PheBT) of rats, we investigated its feasibility and validity and determined effective parameter of the test.Methods Twenty male Sprague-Dawley(SD) weighting 280~290g rats randomized into two groups acute hepatitis rats (n=10) and control rats (n=10). Hepatitis was induced by carbon tetrachloride (CCI₄)olive oil administration through intragastric gavage. PheBT was assisted by small mechanical ventilator improved and air samples were collected discontinuously, 20mg/kg body weight L-1-¹³C phenylalanine (¹³C-Phe)was administered intravenously. Twenty-nine breath samples were taken before and different intervals within sixty minutes after administration. ¹³C enrichment was measured by isotope ratio mass spectrometer.Results All time phase curves of ¹³C enrichment in rat breath reached a peak almost at 2 min after the intravenous administration of ¹³C-Phe. The PheBT parameters, ¹³C excretion rate constant (PheBT-K), of CCI₄ hepatitis rats were significantly lower than that of normal control rats(2.45±0.25)×10^-2 min^-1 vs (2.98±0.19)×10^-2 min^-1, t=5.40, P<0.001. Serum alanine transaminase (ALT), aspartate transaminase (AST), total bile acid (TBA), alkaline phosphatase(AKP) in hepatitis rats were significant higher than that of controls (t value is 8.15, 3.40, 3.90, 4.8 and 4.12 respectively, P<0.05). However ¹³C fast phase disposition constant did not statis-tically differ between the two groups (t=0.58, P>0.05). PheBT-K had significant negative cor-relation with serum ALT, AKP, TBA and total bilirum TBIL (the correlation coefficient r is-0.74,-0.73,-0.82 and -0.67 respectively, P<0.05), and no statistically significant correlation with serum level of AST(r=0.16, P>0.05).Conclusions It was indicated that the small animal breath test model we designed was a virtual tool to use in experimental study on breath test and PheBT-K was a sensitive index.