In vitro fertilization and embryo transfer (IVF-ET) programs use ultrasound extensively for monitoring the growth of ovarian follicles and, subsequently, for confirming the presence of a fetal sac. There have been few reports of the effects of ultrasound on ovulation rates in mammals, and we report here that following exposure to continuous wave ultrasound at a spatial average intensity of 3.0 W/cm2 for five minutes, ovulation rates measured 10 days later were significantly reduced in mice. When temperature elevation of the exposed ovary was measured with a thermocouple, hyperthermia correlated with reduction in ovulation.