The aminopyrine breath test has been performed on Day 1 and Day 7 in pigs following experimental liver allografting. The results show that at 24 hr after surgery there was a significant reduction in liver function in all animals as measured by the elimination rate constant of 14CO2 in the breath. This was very profound in the group that died within 5 days (0.09 ± 0.01 hr−1) and was significantly different from the group that survived (0.16 ± 0.01 hr−1). There appears to be a critical value of the elimination rate constant ( = 0.12 hr−1) which predicts death within 5 days. These results would justify a trial of the aminopyrine breath test in patients after liver transplantation.