The absorption, excretion and metabolism of tulobuterol were studied in various aged rats after oral administration of 14C-tulobuterol. The blood levels of 14C after oral dosing in various aged rats, decreased gradually according to a biphasic process, that is, the first peak appeared at 30-40 min and the second one at 1-2 hr. The excretion of 14C after oral dosing was generally rapid and the radioactivities in the urine and feces for 7 days after oral dosage in various aged rats were 59-66% and 33-36% of the dose, respectively. The biliary excretion of 14C until 24 hr was estimated to be 59-66% of the dose in all aged rats studied, but the rate of excretion in 21-day-old rats was slower than those of 49- and 150-day-old rats. Furthermore, the excretion amount of the ring hydroxylated metabolites, such as 4-hydroxy form (II) and 4-hydroxy-5-methoxy form (IV) into the bile of the 21-day-old-rats was different from those of the 49- and 150-day-old rats. The relation between the activity of metabolizing enzymes in the liver and the age of rats was also studied. The metabolic activity of tulobuterol in the rat liver gradually increased until the 28th day, and thereafter decreased with an increase in the age.
