Pregnant Swiss mice were treated with 0, 5, 10, 20, and 30 mg/kg body weight of bis(tri-n-butyltin) oxide (TBTO) on d 6-15 of gestation. At birth litters were normalized to eight pups, and postnatal evaluation of pup growth rate and behavioral observations of dams were carried out. Litters were sacrificed on postnatal days (pnd) 7, 14, and 21, to perform hematological analysis, in connection with another study. Dam weight gain was impaired in all the treated groups (except at the lowest dose level) in the late phase of gestation. A high incidence of anticipated or delayed parturitions, without any correlation with fetal mass, was observed in the treated groups. All the treated dams showed a significant increase in resorptions, and a decrease in body weight gain between gestational day (gd) 6 and pnd 1. At birth, only the 20 and 30 mg/kg dose groups showed reduced litter size and reduced pup weight. Body weight gain reduction of pups persisted in wk 1 of life only in the 10 and 20 mg/kg dose groups. In addition, the maternal weight trend was affected during the lactation period in the higher dose groups. Postnatal death rate and growth rate of treated pups were affected by an altered maternal behavior; pups, apparently viable and with normal weight, were found often scattered throughout the cage with signs of wounds, and the percentage of dams that had not built a nest increased in the 10, 20, and 30 mg/kg dose groups. Total absence of parental care was noted in many litters, and many infanticidal events were reported. Our results seem to confirm low TBTO embryofetotoxicity, and strongly support the assumption that TBTO's toxicity to the mother is much stronger than its embryo-fetotoxic potential. Most of the reproductive parameters examined in this study were unaffected in the low-dose group, while some indices, such as gestation length and maternal weight gain between gd 6 and pnd 1, were markedly altered also at the 5 mg/kg dose level and appear to be sensitive parameters in assessing maternal toxicity.