Single subcutaneous doses (25 mumol/kg body weight) of tin-protoporphyrin (TP), a potent competitive inhibitor or heme oxygenase (HO), were administered to both suckling and adult Wistar rats. The effect of TP on the carbon monoxide excretion rate (VeCO), an index of total bilirubin formation, and on in vitro carbon monoxide (CO) production by the small intestine were evaluated. Whereas the VeCO of the adult group was decreased (p less than 0.0005) after TP, that of the suckling rat was unchanged. Gradients of CO production along the small intestine were observed in sucklings as well as adults; however, these gradients were in opposite directions. Intestinal CO production was greatest in the adult duodenum, decreasing distally; conversely, the CO production was greatest in the suckling ileum, decreasing proximally. No significant difference in CO production between control and TP-treated adult intestinal mucosa was observed. In sucklings, a significant reduction of intestinal CO production in the TP-treated rats was detected in the duodenum only (p less than 0.05). The results suggest that suckling rats differ from adults in terms of the capacity to produce CO and the direction of the gradient of CO production along the intestine. We conclude that (1) TP may not substantially decrease the in vivo production of CO by the small intestine at a dose which inhibits hepatic and splenic heme oxygenase, and (2) because after a heme load, heme is excreted into the intestine after TP administration, heme-degrading, CO-producing processes in the intestine may contribute to an animal's VeCO under such conditions.