Male Sprague-Dawley rats were exposed for 6 h per day for nine days to pulse-modulated microwave radiation (1.3 GHz, at 1-microseconds pulse width, 600 pulses per second). Exposures were carried out in cylindrical waveguide sections at a mean dose rate of 6.3 mW/g; sham controls were treated similarly and received no irradiation. At time periods corresponding to 0.5, 1.0, 2.0, and 4.0 cycles of the seminiferous epithelium, groups of four sham-irradiated and four irradiated rats were killed and the testes removed for analysis. Net mass of the testes, epididymides, and seminal vesicles; daily sperm production (DSP) per testis and per gram of testis; sperm morphology; and the number of epididymal sperm were determined. There were no statistically significant differences between the sham-irradiated and irradiated groups with respect to any measured variable. In a group of seven surrogate animals of similar body mass, the dose rate of 6.3 mW/g caused a net change in body temperature (via rectal probe) of 1.5 degrees C.