Groups of rats were exposed in utero from day six of gestation through 40 to 41 days of age for four hours a day in a temperature- and humidity-controlled environment under far-field conditions to 2450-MHz (CW) in an electrically anechoic chamber. An equal number of sham-exposed animals, maintained under the same environmental conditions, served as controls. Two experiments have been performed at this frequency at a power density of 5 mW cm−2. Specific absorption rates (SARs) for rats of different ages were determined by twin-well calorimetry and were found to range from 0.7 to 4.7 mW g−1. Rats were weighed at selected intervals to determine if microwaves affect growth. At 20 to 21 and 40 to 41 days of age, rats were bled and complete blood-cell counts were done. In addition, the in vitro blastogenic response of blood and lymph-node lymphocytes were measured by 3H-thymidine incorporation into DNA following stimulation of cells with T- and B-lymphocyte mitogens. There were significant increases in the mitogen-stimulated response of both T- and B-lymphocytes from irradiated rats. There were no consistent changes, however, in peripheral blood-cell counts after exposure at this frequency. These results indicate that long-term exposure of rats in utero and through early life may result in increased response of lymphocytes to stimulation with mitogens in vitro.