Circadian variations in microsomal drug-metabolizing enzyme activities in rat and rabbit tissues.

1. The circadian variations in drug-metabolizing enzyme activities have been measured in microsomal fractions from the livers, lungs, and intestines of rabbits and in the livers and intestines of rats. 2. Circadian rhythms in enzyme activities in the rat intestine show 2 peaks in activity (around 0500 and 1900 h) with two intermediate troughs around 1200 and 2400 h. This twin-peaked (biphasic) profile contrasts with the single maximum and minimum in activity observed in rat liver. 3. Rabbit hepatic, pulmonary, and intestinal microsomal drug-metabolizing enzyme activities also show a biphasic profile during a 24 h period. This profile is less distinct in liver than in intestine, and less distinct in intestine than in lung. Consequently, rat and rabbit hepatic drug metabolizing enzyme activities show a considerable species difference with respect to their circadian profiles in activity.