The livers of 6 Gunn rats (200 gm.), 3 jj and 3 Jj, were perfused for 1 hr. with std. KBR soln. to which bilirubin in conc. of 20 mg/dl was added to 4. One jj and 1 Jj were perfused in the dark, 2 each in blue fluorescent light (420-470 nm, 18 μw/cm2/nm) Bilirubin was extracted in CCl4 from half of each liver, the other half analyzed for activity of: cytochrome P450, b5, p-nitrophenol glucuronidation, benzo(a)pyrine and analine hydroxylation, aminopyrine demethylation. All livers perfused with KBR-bili. or KBR alone under light had little or no bilirubin at end of perfusion. Those perfused in the dark had significant amounts of bilirubin. Assays of both jj and Jj rat livers showed a marked increase in activity of all 6 enzymes, 40+ percent. As all rats were female, no intersex variable was encountered. Determination of bilirubin conc. of outflow KBR-bili. perfusate showed a small but significant decline q5 min. during perfusion. Bilirubin extract from livers of both jj and Jj rats in the dark was in high conc. We conclude that visible light penetrates through the intact rat liver, directly photodegrading bilirubin, possibly enhancing its uptake. These data further indicate that visible light (420-470 nm) induces hepatic cell enzyme activity. The possibility exists that visible light irradiation, as in phototherapy, will enhance drug metabolism by hepatic cell enzyme induction.