A single dose of 25 mg/kg methylmercuric chloride (MeHg) was given orally to gravid ICR mice. Cleft palate was induced in 100% of the offspring, with the critical treatment period ranging from day 10/8 hours (10/8) to 12/16 of gestation. Dose-dependent body weight reduction was observed in day 18 fetuses from both the day 10/8 and 12/16 groups. However, fetal weight reduction was greater in the day 12/16 group for all the MeHg treatments investigated. The relative potency of the induction of cleft palate by MeHg was slightly but significantly higher in the fetuses of the day 12/16 group (1.044-1.197-fold in 95% limits) than in the day 10/8 group. The results showed that when 25 mg/kg of MeHg was given to the fetuses in the day 10/8 group, palatal shelf growth was delayed at a more primitive stage than in the day 12/16 fetuses. Moreover, disharmony of development between the overall fetus and palatal shelf was noticed. Furthermore, in the day 12/16 fetuses, a delay of palatal shelf growth occurred just prior to shelf elevation. Prior to shelf elevation, coordination was probably lost in the development between the fetus and the palatal shelves. Normal palatal closure in ICR fetuses occurs about 1 day and 10 hours earlier (P less than 0.05) than in the A/J fetuses (Biddle, '80). Normal palatal shelves in ICR fetuses moved rapidly, with 3.0 to 5.7 hours (in 95% limits) required for all fetuses to achieve elevation, while, in MeHg-treated groups, palatal shelf elevation did not occur. The results suggest that the cause of the failure in palatal shelf elevation may be understood by examining the disharmonious development of the fetus after exposure to MeHg.