An optoelectronic instrument for detecting the ovum and its surrounding cumulus mass in the oviductal ampulla of rabbits was successfully tested in the abdominal dish preparation of Blandau. An optical transducer, constructed in the shape of a cuff designed to fit about the oviduct, measures changes in the intensity of light transmitted through it. As a cumulus egg mass passes through the sensor, a large sustained increase in light transmittance is recorded. Since ampullary egg transport follows ovulation within minutes, this instrument may provide a means of accurately determining the time of ovulation.