Urinary excretion of aspirin.

Six human volunteers were each given single oral doses of aspirin (ASA) ranging from 300-1,500 mg. The unchanged ASA excreted in the urine was proportional to dose and urinary pH. The mean percent (+/- s.d.) of dose excreted was 1.9 +/- 0.67. The clearance for ASA was 1.42 +/- 0.28 1/h. The rate of in vitro hydrolysis of ASA to salicylic acid in urine at 37 degrees C was 4 micrograms/min for an initial ASA concentration of 7.5 mg in 100 ml human urine.