A miniature scintillation detector recently designed and constructed in this laboratory can be inserted into the esophagus and under fluoroscopic vision positioned so that radioactivity emitted from the blood in the descending aorta can be detected and a time-concentration curve recorded from which cardiac output can be determined. Twenty simultaneous determinations of cardiac output by the isotope-dilution method with the esophageal scintillation counter probe and the dye-dilution technique were done in 10 dogs. There was a good correlation between the two methods. Among the technical advantages of the isotope-dilution method over the dye-dilution method are avoidance of arterial puncture, only one sample of venous blood required, an easily performed procedure, and ability to permit repeated determinations of cardiac output with a minimum of manipulation and trauma.