Natural products obtained from various plant materials including terrestrial plants play an important role in chemistry of drugs. They provide amazing source of new drugs leading to development of new chemical entities for further drug development. In the course of study an attempt was made for the identification of biologically active constituents present in the ethanolic extract of Dalbergia sissoo leaves and bark and evaluation of both extracts for antipyretic activity through Brewer’s yeast–induced pyrexia in Albino Wistar Rats. This antipyretic activity is related to the inhibition of prostaglandin synthesis. Results showed significant antipyretic activity in pyrexia in rats throughout the observation period of 5 hrs. The Test agent or co-solvent was administered orally and the temperature was measured at 1, 2, 3, 4, and 5 hrs. After drug administration.