Hepatic microsomal heme oxygenase, the rate-limiting enzyme in heme degradation, is rapidly and markedly stimulated in selenium-deficient rats but not in controls, after a single injection of phenobarbital sodium. This stimulation occurred as early as 2 h and reached an 8- to 10-fold maximum 6 h following the drug. These observations suggest that phenobarbital rapidly and markedly enhances the degradation of hepatic heme in selenium deficiency. The cause for this rapid phenobarbital-mediated stimulation of heme degradation was investigated. It could not be ascribed to either accelerated turnover of cytochrome P-450, or enhanced lipid peroxidation possibly resulting from the concomitant lack of glutathione peroxidase in selenium deficiency. For these reasons, the metabolism of hepatic heme in the selenium-deficient rat liver was further examined during the 6-h period following phenobarbital. These studies indicated that whereas heme was synthesized much faster and to a greater extent in the selenium-deficient rat liver than in the control, its utilization in the formation of hemoproteins such as cytochrome P-450 and tryptophan pyrrolase was apparently impaired in selenium deficiency. Such defective utilization of heme in the 6-h period following phenobarbital could effectively result in a relative excess of unutilized "free" heme; hence the consequent stimulation of microsomal heme oxygenase for its disposal.