Near the beginning of the second trimester of pregnancy, 33 squirrel monkeys were exposed to 2450-MHz irradiation in a multimode cavity at whole-body average specific absorption rates equivalent to those resulting from exposure to plane wave irradiation at 0.034, 0.34, and 3.4 W/kg; exposed monkeys were compared with eight pregnant sham-exposed monkeys. Eighteen of the irradiated mothers and their offspring were exposed for an additional 6 months after parturition, and then their offspring were exposed for another 6 months. No differences were found between irradiated and control adults with respect to the number of live births produced or to measures of locomotor activity, maternal care, urinary catecholamines, plasma cortisol, 3H-thymidine and 14C-uridine uptake by phytohemagglutininstimulated blood lymphocytes, or electroencephalographic (EEG) activity. Similarly, no differences were found between exposed and nonexposed offspring on the same blood, urine, and EEG parameters. Growth rate and most aspects of behavioral development were not altered by exposure. The major difference between irradiated and control offspring was the high mortality rate (4/5) before 6 months of age in those exposed at 3.4 W/kg both before and after birth. These results indicate that microwaves at power densities to 3.4 W/kg might have little direct effect on the monkey fetus when exposures occur in utero during the latter half to two-thirds of pregnancy, but that continued exposure after birth might be harmful.