Paraquat (1, 1'-dimethyl-4, 4' bipyridilium salt) induced heartwood formation in Azadirachta indica. The wood at the site of treatment showed desiccation. The induced heartwood is observed even at the height of 3 to 3.5 metres from the site of the treatment. Histochemical studies showed disappearance of starch grains and accumulation of lipids, insoluble polysaccharides and phenolics in the treated wood. The axial and ray parenchyma cells at the sapwood-heartwood boundary in the treated wood showed enhanced acid phosphatase, ATPase and succinate dehydrogenase activities. Traumatic gum ducts were also observed in the treated wood.