An extract of Ruta graveolens was found to be cytotoxic to Dalton’s lymphoma ascites (DLA), Ehrlich ascitescarcinoma (EAC) and L929 cells in culture (IC100 = 16mg/ml) and also to increase the lifespan of tumour bearinganimals. The extract further decreased solid tumours developing from DLA and EAC cells when given simultaneouslywith elongation of the lifespan of tumour-bearing animals. A homeopathic preparation of Ruta graveolens (200c) wasequally effective. Neither was effective for reducing already developed tumours. The Ruta graveolens extract wasfound to scavenge hydroxyl radicals and inhibit lipid peroxidation at low concentrations. However, at higherconcentrations the extract acted as a prooxidant as inhibition of lipid peroxidation and scavenging of hydroxylradical was minimal . These data indicates that the prooxidant activity of Ruta graveolens may be responsible for thecytocidal action of the extract and its ability to produce tumour reduction.