A new concept for determining relative blood flow in single arteries using videodensitometry to obtain contrast dilution curves has been experimentally evaluated. Flow as a fraction of cardiac output in the cerebral, renal, splanchnic, and iliac arteries was determined in 110 measurements in 11 dogs. The mean difference between measurements by electromagnetic flow probe and videodensitometric values was less than 2%, and the correlation coefficient was r = 0.96 for all data. The method is simple compared to previous videodensitometric techniques, because the dimensions of the vessel and the mean transit time of the contrast bolus can be neglected. The necessary equipment includes a stable image intensifier-television chain, a videotape recorder, a videodensitometer, and a graphic recorder. The method is now used to measure blood flow in cerebral, coronary, splanchnic, renal, and ileofemoral arteries during routine clinical angiography.