Butyltin trichloride (BT), dibutyltin dichloride (DBT) and tributyltin chloride (TBT) were compared for their developmental toxicity including teratogenic potential following administration during the susceptible period for the teratogenesis of DBT. Pregnant rats were given either BT at a dose of 1000, 1500 or 2000 mg kg-', DBT at a dose of 10 or 15 mg kg-I or TBT at a dose of 40 or 80 mg kg-' by gastric intubation on days 7 and 8 of pregnancy. Although maternal toxicity occurred, as evidenced by a significantly increased maternal lethality at 1500 and 2000 mg kg-I and decreased maternal weight gain at lo00 and 1500 mg kg-', no significant increase in the incidences of postimplantation loss and malformed fetuses were observed after treatment with BT. Treatment with DBT resulted in a significantly lower maternal weight gain, lower fetal weight and higher postimplantation embryolethality. A significantly and markedly increased incidence of fetuses with malformations, such as exencephaly, cleft jaw, cleft lip, ankyloglossia, club foot, deformity of the vertebral column in the cervical and thoracic regions and of the ribs and ano-or microphthalmia, was observed in both groups treated with DBT. While treatment with TBT at 40 and 80 mg kg-' caused a significantly decreased maternal weight gain and increased postimplantation embryolethality, no significantly increased incidence of malformed fetuses occurred. It could be concluded that BT, DBT and TBT are different in the susceptibility and spectrum of developmental toxicity.