UDP glucuronyl transferase (bilirubin) (UDPGT) was studied in liver homogenates of pregnant and nonpregnant adult female rats. No change in maternal liver UDPGT activity was observed during pregnancy. Administration of phenobarbital (PB) for the 3 days preceding enzyme assay increased UDPGT activity in nonpregnant rats by 61.8%. The same treatment of pregnant rats produced a similar increase in UDPGT activity measured on the 5th, 7th and 9th day of gestation. After day 9, the extent of this PB-mediated increase in hepatic UDPGT activity progressively decreased, and by the 21st day of gestation the increase was only 19.0 %. This observation of reduced response of UDPGT (bilirubin) after PB administration during late pregnancy in the rat, may be significant in understanding the mechanism of neonatal glucuronyl transferase deficiency as well as drug metabolism in mother and fetus.