Impregnated Swiss mice were irradiated 2 h daily on days 1-18 of gestation in nonthermal (1 or 10 mW/cm2) or thermogenic (40 mW/cm2) 2,450-MHz microwave (MW) fields. On the 19th day of pregnancy all dams were killed to check the number of resorptions. Living fetuses were isolated, weighed and checked for the presence of macroscopically visible malformations of skeleton and cleft palate (CP) and/or lip (CLP). Some of the pregnant mice in each group were injected intraperitoneally on the 9th day of gestation with 10 mg/kg of cytosine arabinoside (ara-C), a well-known teratogen resulting under the above conditions in the appearance of about 15% (42 of 354) of resorbed implantation points and the development of CP or CLP in about 30% (96 of 312) of fetuses. Exposure to nonthermal MW fields during pregnancy did not lead to resorptions or detectable malformations; however, the body mass of 19-day fetuses was significantly lower than in sham-irradiated controls. MW hyperthermia (40 mW/cm2) applied during pregnancy led to an increased number of resorptions - about 25% (37 of 157) compared to 2% (6 of 306) in controls. Significant enhancement of the teratogenic potency of ara-C was observed after combined exposure to both ara-C and MWs during pregnancy. In dams treated with ara-C alone about 20% (62 of 358) resorptions and about 30% (91 of 296) fetuses with CL or CLP were found; additional exposure in MW fields (10 mW/cm2), not resulting per se in resorptions or detectable malformations, increased the numbers to 45% (213 of 448) of resorptions and to 70% (167 of 235) fetuses with CL or CLP.+