Toxic effects of yellow oleander (Thevetia neriifolia Juss) seed kernels were evaluated against the roof rat (Rattus rattus Linn). Crushed ground seed kernels were fed with bait at 20 and 30% concentrations. The bait was fed up to mortality or for a maximum of 10 d. Major signs of poisoning observed were hind limb paralysis, rolling of the body on the long axis, circular flailing of the tail, muscular twitch, tetanic convulsions, tremors, collapse and death. Significant reductions in the rats' weights were observed. The observed mortalities were 16/20 and 18/20 with the above respective doses. Statistically significant reductions in hemoglobin, red blood cell count, total leucocyte count and neutrophils, and increased lymphocytes were observed. Reductions in blood glucose and serum proteins, and increased lymphocytes were observed. Reductions in blood glucose and serum proteins, and increased BUN, SGOT and LDH, were also significant. Histopathological studies showed inflammatory and degenerative changes in the liver and kidney. Severe to moderate fatty metamorphosis, congestion, hepatocytolysis, nuclear degeneration, pyknosis, and necrosis were major changes in the liver. Proliferation of glomerular endothelium, hypercellularity of the glomerulus, necrosis of convoluted tubular epithelium, disappearance of nuclei and pyknosis were important changes in the kidney cortical region. Atrophy, erosion and inflammatory changes were observed in the stomach mucosal linings.