Antipyretic testing of aspirin in rats

A reproducible antipyretic test method has been developed to assay low doses of po administered aspirin in rats. Rectal temperatures recorded 19–24 hr after a sc injection of 20 ml/kg of an aqueous suspension of brewer's yeast were elevated 1.5 to 2.0°C. Statistically significant antipyresis was demonstrated 15 min after a po dose of 150 mg/kg of aspirin. Maximum effect occurred 120 min post drug. Aspirin, administered po at 5, 25, 50 and 100 mg/kg, exhibited antipyretic activity at all doses. The ED50 values calculated from these data ranged from 10.3 to 21.1 mg/kg. Untreated fevered controls showed no marked temperature variability during the test period.