Monthly variations in the clearance of antipyrine in the rat

The variation in the pharmacokinetics of antipyrine was determined in synchronized rats over a period of 12 consecutive months. The mean values +/- SE (range) for the serum half-life, apparent volume of distribution and total body clearance of antipyrine were: 2.5 +/- 0.6 (1.6-4.9) h, 868.7 +/- 203.9 (590-1275) ml/kg and 269.2 +/- 14.7 (165-516) ml/h/kg respectively. The magnitude of the variation was three fold for the serum half-life and body clearance and two fold for the apparent volume of distribution. A new gas-liquid chromatographic assay for antipyrine is also described.