Monocrotaline, a pyrrolizidine alkaloid, caused changes in most of the biochemical parameters in rats 12 days after a single dose of 120 mg/kg. These included significantly increased activities of hepatic succinate dehydrogenase, acid ribonuclease, acid phosphatase, gammaglutamyl transpeptidase and 5'-nucleotidase and decreased in the activities of glucose-6-phosphatase and cytochrome P450. The levels of DNA, RNA and glycogen in liver and albumin and protein in serum decreased while serum bilirubin increased. The histopathological changes in liver were characterized by diffused hepatocyte alterations in the form of ballooning, granular cytoplasm, indistinct cell outlines, nuclear changes, focal necrosis, and vascular damage. When picroliv, a standardized iridoid glycoside fraction of Picrorhiza kurroa, was administered orally in a dose of 25 mg/kg simultaneously with monocrotaline, alterations in most of the biochemical parameters along with the histopathological changes in liver caused by monocrotaline were prevented.