reaching 10 times adult values by day 18. If 50 mg of drug is injected immediately incubation begins, activity of the enzyme is readily detectable in the visceral area of the embryo within 96h of incubation. Phenobarbital also overrides the natural regulation of UDP-glucuronyltransferase in extrahepatic tissues. We have found that organ culture of 11-day embryo kidney, and possibly gut, permits precocious rise of the transferase in these tissues; in ouo, as found by Dutton & KO (1966), activity of kidney and gut remains virtually zero until after hatching. Phenobarbital injection into the egg markedly increases the transferase in embryo kidney; again, increase is much less obvious in embryo gut. So far we have no evidence of transferase activity or of overall glucuronidation in the yolk-sac, chorioallantoic and amniotic membranes, with or without previous injection of phenobarbital. Injection of phenobarbital in ouo also increased transferase activity towards p-nitrophenol, p-nitrothiophenol and oestriol. Injection of up to 2.5mg of p-nitrophenol (pH7.0) into the air-space of 11-day eggs had no effect on embryo liver transferase assayed 4 days later; higher doses of the substrate proved toxic. We have found that I ,2and 3,4-benzopyrene, as well as barbiturate drugs, increase the rate of development of UDP-glucuronyltransferase activity in chick-embryo liver towards o-aminophenol when added to the medium in which the liver segments are being cultured. Injection of over 2.5mg of 1,2-benzopyrene into 8-day eggs proved toxic; smaller dosages have so far failed to reproduce the marked effect on the enzyme of phenobarbital in ouo.