Fetuses of pregnant CD-1 Swiss albino mice, exposed to 0.1% nicotine sulphate at a dose of 1.67 mg/kg body weight from the 6th to the 15th gestational day, were compared with control fetuses to assess the effects of nicotine on first molar odontogenesis. Mothers were sacrificed on the 18th day of gestation. The 130 nicotine treated fetuses, as well as the 348 control fetuses were embedded in paraffin and sectioned in the frontal plane. The developing molars of the experimental fetuses were retarded, less differentiated, and reduced in breadth in comparison with controls. The developing molars of the control fetuses were in the bell stage of odontogenesis, whereas those of the experimental population were either in the late cap or early cap stage, depending on the absence or presence of palatal cleft, which occurred in 9.6% of the fetuses. It is suggested that nicotine, or its metabolic byproducts, interfere with normal interaction between the epithelial and mesenchymal components of the developing tooth.