Using exposure conditions comparable to those which have been associated by others with positive results, we tested for effects of ultrasound on sperm production over a period of 12 weeks following treatment. Continuous wave exposure with spatial average intensities of 1, 2 and 4 W/cm2 and exposure times up to 10 minutes were used. In some experiments, the exposures were repeated after an elapsed time of 48 h. No significant changes in spermatogenesis were related to any of the exposure conditions in spite of the fact that some of the treatments caused thermal tissue damage near bone. No effects of exposure were found in weights of the testis, prostate, seminal vesicle, or whole body.