Primigravid rats were exposed for 20 min to microwave radiation (2450 MHz; ) in a multimode cavity or to infrared radiation in an incubator ( ∼ 47 °C). They were exposed during one of seven days of gestation, the 10th through 16th. The Δ of the dams' colonic temperature, about 3.5 °C, was equated in the microwave and infrared treatments. Control groups of primigravid dams were sham radiated. During the 19th day of gestation, fetuses were taken by Caesarian section and were examined for structural abnormalities and for insult. In addition, fetal brains were analyzed for levels of dopamine (DA) and norepinephrine (NE). Findings: (1) the incidence of lethal dosing was 27% and 12%, respectively, in microwave- and infrared-irradiated dams; (2) the averaged numbers of fetal resorptions in infrared- and sham-irradiated dams are low (both ∼2%) but the number is elevated nearly sixfold in microwave irradiated dams; (3) the incidence of mortality in nonresorbed fetuses was nil; (4) extensive hemorrhagic signs were observed in both microwave- and infrared-irradiated fetuses but there was no discernible evidence of structural malformation; (5) averaged fetal mass is slightly (10%) but reliably smaller in both irradiated groups as compared with controls; and (6) averaged whole-brain DA levels do not differ reliably as a function of treatment but the averaged level of NE in brains of microwave irradiated fetuses is reliably below those of infrared and sham-radiated fetuses. While the averaged ΔTs of the two groups of irradiated dams are equal, increments of temperature of microwave irradiated dams were more variable; correlative analysis provided evidence that mortality and resorption are probably a function of peak body temperature irrespective of source of radiation.