Four experiments were performed in which six pregnant rats were exposed from day 12 of pregnancy to parturition, for 4 hours a day in a temperature-controlled environment, to 425-MHz (CW) radiation, using a multimode rectangular strip transmission line. Four male pups born to each dam were subsequently irradiated under the same RF exposure condition for 20-21 days of age (2 pups) and 40-41 days of age (2 pups). Specific absorption rates (SARs) for rats of different ages were determined by twin-well calorimetry as well as from calculations of power measurements of incident, reflected, and transmitted energy. Values of SARs between 3.1 and 6.7 mW/g were obtained for rats so exposed at 425 MHz. At selected times, rats were weighed to determine if the irradiation affected growth. Two rats from each litter (4 pups) were euthanized at 20-21 and two at 40-41 days of age and blood was obtained for complete blood counts. The in vitro blastogenic response of blood and lymphnode lymphocytes was measured by H-thymidine incorporation into DNA following stimulation of cells with T- or B-lymphocyte mitogens.