The fetal heart was located by real-time “B” scanning and studied by means of conventional time-motion echocardiography. Estimates of left ventricular output were made in the 13 fetuses and 11 neonates with average values of 109 ml/kg/min in the fetuses and 157 ml/kg/min in the neonates. Diastolic thickness of left ventricular wall was generally 6 mm. These results are in agreement with measurements obtained by invasive means and suggest that ultrasound may play a role in the study of fetal and neonatal cardiac physiology and pathology.