OBJECTIVE To describe the kinetics of demethylation of 13C-aminopyrine in healthy dogs for use in determining the most appropriate time for collection of blood samples for a 13C-aminopyrine demethylation blood test for evaluation of hepatic function. METHODS 9 healthy dogs. METHODS A 2-mL baseline blood sample was collected into an evacuated heparinized tube, and 13C-aminopyrine was administered to each dog (2 mg/kg, IV). Additional 2-mL blood samples were collected 15, 30, 45, 60, 75, 90, 105, 120, 135, 150, 180, 240, 300, and 360 minutes after 13C-aminopyrine administration. The CO2 was extracted from blood samples by addition of a strong acid, and the percentage dose of 13CO2 (PCD) in the extracted gas was determined by fractional mass spectrometry. RESULTS No dogs had gross evidence of adverse effects, and all had an increase in PCD after IV administration of 13C-aminopyrine. The PCD had the least variability among 5 variables used to evaluate hepatic demethylating capacity. Peak PCD was detected at 30 minutes in 1 dog, 45 minutes in 5 dogs, 60 minutes in 2 dogs, and 75 minutes in 1 dog. The mean PCD for the 9 dogs peaked at 45 minutes after 13C-aminopyrine administration. CONCLUSIONS PCD appears to be the preferable variable for evaluation of hepatic demethylating capacity. Intravenous administration of 13C-aminopyrine leads to a consistent increase in PCD. Mean PCD peaked 45 minutes after administration, suggesting that blood sample collection 45 minutes after 13C-aminopyrine administration may be appropriate for use in estimating hepatic demethylating capacity.