[en] This paper describes studies of the performance of a detector system for the measurement of "8"6Rb in the myocardium and "1"3"1I-HSA in the heart chambers in studies of coronary blood flow and cardiac output. The system comprised three twin-counter units each consisting of a pair of opposed collimated scintillation detectors coupled to an amplifier, pulse-height analyzer, sealer and digital ratemeter. Collimation was provided by exchangeable cylindrical collimators of two different sizes supplemented by adjustable lead shielding. Studies of the performances of the system, in particular as regards constancy of counting efficiency within the field of view of the detector system were carried out with "8"6Rb and "1"3"1I sources mounted within Perspex phantoms. The sources used included point sources, aqueous solutions contained in spherical flasks of various sizes and aqueous solutions contained in a Perspex model of the heart divided into nine compartments. Studies were carried out using both twin-counter units and single counters. The results indicated that the performance of the twin counter units was satisfactory except for measurements on markedly enlarged hearts. Contribution from radioactivity in adjacent organs or regions could be satisfactorily reduced by the use of the supplementary lead shielding. A single frontal counter, on the other hand, was found to have a counting efficiency which varied rapidly with source position and so to be generally unacceptable. The paper reports preliminary trials of the system on patients. (author)