Tests of mutagenesis and reproduction were conducted in male rats which were irradiated by 2,450-MHz, continuous-wave (CW) microwaves, 4 hr/day from day 6 of gestation to 90 days of age at 5 mW/sq. cm.; or 5 hr/day for five days beginning on the 90th day of age at 10 mW/sq. cm.; or 4 hr/day, 5 days/wk four weeks, beginning on the 90th day of age. During selected weekly periods after treatment, the rats were bred to pairs of untreated, normal female rats that were examined in late pregnancy by means of the dominant lethal assay. The reproductive efficiency of these males, as reflected in their breeding, was also examined for changes relating to their microwave experience. No significant experience of germ-cell mutagenesis was detected when data of microwave-exposed males were compared with those of sham-exposed males, even though there were significant increases in rectal and intratesticular temperatures at a power density of 28 mW/sq. cm. Temporary sterility, as indexed by fewer pregnancies, was seen at the highest power density.