The visual interpretation of Thallium-201 myocardial images is made more difficult by nonhomogeneity of myocardial tracer uptake in normal studies. To increase the objectivity of the study, the variation in count density in different myocardial areas has been calculated using a computer regions of interest technique in 14 healthy, young subjects following intravenous injection of the radionuclide at rest and in 10 after injection during maximal exercise. The "normal ranges" for regional Thallium-201 uptake so obtained are presented and discussed.