Pretreatment of rats with a single dose of polychlorinated biphenyls caused an increase in the activity of UDP glucuronyltransferase (EC 18.104.22.168; acceptor unspecific) in liver microsomes towards the substrates p-nitrophenol and 4-methylumbelliferone. The results of kinetic experiments suggest enhanced enzyme activity is caused by enzyme induction. The effect was dependent on dose and chlorine content of the polychlorinated biphenyls administrated. The time course of UDP glucuronyltransferase activity after a single dose of polychlorinated biphenyls demonstrated a long lasting effect. Elevated biliary excretion of sulfobromophthalein conjugates after pretreatment with polychlorinated biphenyls was also demonstrated. In addition bile flow per g liver was increased and sulfobromophthalein concentration in serum and liver was reduced. Maximum effects were observed three days after treatment; thereafter these parameters slowly returned to normal levels.