Bilirubin, the yellow pigment of jaundice, is a linear tetrapyrrole with a methylene group at its center, C(10), a position of crucial importance to its conformation and metabolism. The presence of the central methylene group allows the bilirubin to fold into an intramolecularly hydrogen-bonded conformation. This paper describes the first synthesis of a bilirubin analogue with an oxo group at C(10). The change from CH2 to C=O, from sp(3) to sp(2), is designed to stress the molecule at its hinge and relax its conformation. Such compounds have been suggested as potential oxidative metabolites of bilirubin in vivo. 10-Oxo-mesobilirubin-XIII alpha (1) is a red crystalline solid, unlike its parent, mesobilirubin-XIII alpha, which is a bright yellow solid. It is surprisingly polar, relative to the parent, yet it does not exhibit a significantly larger bicarbonate/chloroform partition coefficient. Like the parent, 1 appears to adopt an intramolecularly hydrogen-bonded ridge-tile-like conformation. In normal rats, 1 is metabolized to acylglucuronides, which are secreted into bile, but a portion of the administered dose is secreted into bile intact. In mutant rats (Gunn rats) lacking bilirubin glucuronyl transferase, 1 was excreted efficiently in bile in unchanged form, unlike the parent with a methylene group at C(10). Thus, introduction of the oxygen function at C(10) has little effect on hepatic uptake but a dramatic effect on canalicular secretion into bile.