When isolated hamster lungs were perfused in a recirculating system for 2 hrs, 4-methylumbelliferone from the perfusion medium was conjugated in perfused lungs with endogenous UDP glucuronic acid to 4-methyl-umbelliferylglucuronide. When hamsters were exposed to cigarette smoke during one hour in an inhalation chamber 20 hrs before the perfusion, the rate of glucuronide conjugation was slightly increased. When isolated lungs were ventilated intermittently with cigarette smoke during the perfusion, the rate of glucuronide conjugation was decreased.