14C-labelled biliverdins IX alpha, beta, gamma and delta have been prepared in vitro from haemoglobin obtained from duck erythrocytes incubated with 5-amino[4-14C]-laevulinic acid. When injected intravenously into rats with biliary fistulae, about 60% of the label was recovered in the bile in 24 h after the alpha isomer was given, while approximately 10% was recovered with injection of the beta isomer. The gamma and delta isomers gave intermediate values. In each experiment, most of the recovered isotope was found in association with conjugated bile pigment. Thus, the metabolic pathway for bile pigment excretion in the rat handles the IX alpha isomer preferentially but is not specific for it.