We studied the effect of tin protoporphyrin (TP) on bilirubin production in adult Wistar rats by quantifying in vivo carbon monoxide (CO) excretion and the simultaneous excretion of biliary heme after common bile duct cannulation. A known amount of heme was injected intravenously as red blood cells (RBC) damaged with a sulfhydryl inhibitor, N-ethylmaleimide. The recovery of heme as CO or biliary heme in the cannulated animals was calculated as the molar percent of heme recovered over heme injected. For cannulated controls (n = 4), the recovery was 89 +/- 6% SD, and no heme appeared in bile. Cannulated rats treated with TP (n = 4) had 64 +/- 11% recovered as CO and 30 +/- 11% as heme in bile. Our findings suggest that TP is an effective in vivo inhibitor of exogenous heme catabolism and bilirubin production in adult rats. Furthermore, this inhibition results in increased excretion of heme into the bile proportional to the degree of inhibition.