Dimethyltin dichloride (DMDC) is commonly used as a stabilizer in PVC pipe used for transport of potable water. Learning deficiencies have been observed postnatally in pups from DMDC-treated dams. Studies were conducted with female Sprague-Dawley rats to determine whether DMDC was absorbed by the dam and transferred across the placenta to fetal blood and brain tissue. This was accomplished in three phases: (1) a comparison of absorption of organic and inorganic tin from drinking water, (2) a comparison of prenatal and postnatal levels of tin in the pups in cross-fostering studies, and (3) a (14C)dimethyltin dichloride tracer study to determine whether organic tin passed to the pup intact. Major findings include: (1) DMDC is absorbed in the gastrointestinal tract of the dam much more rapidly than Sn2+; (2) the more rapid absorption of DMDC results in higher concentration of tin in fetal blood and brain; and (3) in fetuses that receive tin as DMDC, both tin and the methyl carbon are absorbed by the dam and transferred to the blood and brain of the fetuses.