This study aims to explore the effects of the organ chlorine pesticide (OCP) endosulfan on the developing kidney types of the chick embryos. Embryos aged 24 hours were treated with a single dose of 7 or 14 or 21 mg/egg (¼ or ½ or ¾ of the endosulfan LD50, respectively) through the egg air space. The eggs were opened on embryonic days (EDs) 6 and 12 and the embryos were anesthetized, dissected out and the lumbar areas processed for paraffin embedding, stained with haematoxylin and eosin, and the kidney types were examined by light microscopy. In control embryos of ED 6, the only observed excretory tissue was of the mesonephric type. On ED 6, the endosulfan treatment had resulted in dose-dependent histopathological lesions included: a general delay of excretory units differentiation, reduction in the numbers of mesonephric ducts and tubules, irregular distribution of renal corpuscles, high rate of degenerative changes with atrophy and/or vacuolization of epithelial cells and excessive hemorrhage. In controls of ED 12, both mesonephric and metanephric kidneys were observed. The mesonephroi were fully developed, enlarged, and contained much more closely associated components than those of the 6- day-old stage. The effects of endosulfan on ED 12 were also dose-dependent and included increased regressive changes of mesonephroi. These changes were represented cyst-like appearance, dilatation of collecting tubules, necrosis and apoptosis, damaged renal corpuscle, and persistence of primitive excretory units. Also, the metanephros was either poorly developed or did not show any appearance. These findings suggest that endosulfan exhibits severe damaging effects on the developing kidney of the chick embryos.