The purpose of the present study was to describe the technique for and findings of ultrasonographic examination of the rat uterus for diagnosis of early and midterm pregnancy. The uterus of anesthetized Wistar rats was examined between days 9 and 16 post coitum by transabdominal real-time ultrasonography by using a 12-MHz linear transducer. Pulsed-waved color Doppler sonography was used to measure the embryonic heart rate. The embryonic vesicles were detected with 25% false-negative diagnosis on day 9, 8% on day 10, and 0% thereafter. By day 12, the embryos were detected with measurable crown-rump length and heart rate. Ultrasonographic evidence of pregnancy in the rat was present by day 9 post coitum. Diagnosis of pregnancy was confirmed by detection of the embryo heart beat by day 12. Embryo characteristics were ultrasonographically measurable between days 9 and 16.