[Diurnal rhythm of phenacetin and antipyrine elimination in rats].

Pharmacokinetic studies indicate the diurnal rhythmicity of metabolism of many drugs. Partially it is a result of circadian variations of mixed function oxidase system, which metabolizes some drugs. The aim of the experiment was to evaluate the elimination rate phenacetin and antipyrine in different periods of day under standard light/dark (L/D) conditions (08.00-20.00/20.00-08.00). Studies were carried out on rats intragastrically treated with phenacetin in a dose of 0.42 mmol per kilogram of body weight or antipyrine in a dose of 0.17 mmol/kg at 14.00, 20.00, 02.00, 08.00 and 14.00 o'clock. The rate constants (k) were calculated on the basis of concentration of both substances in blood 2 and 3 hours after administration. Excretion of p-aminophenol in urine was determined in mentioned hours after administration of phenacetin in a dose of 0.28 mmol/kg per os. Differences of rates of elimination of both drugs were found. The maximum occurred at 10.00, and minimum at 22.00 o'clock, which indicate circadian periodicity of metabolism.