The first pass of a bolus of radioactivity (99mTc) through the heart and lungs was studied in 27 anaesthetised healthy adult mongrel dogs, using a gamma camera with a computer on-line. Bodyweights ranged from 9 to 60 kg, heart rate from 108 to 150 beats min-1. Quantitative analysis revealed that the distribution volume (DV) of the labelled blood, the cardiac output (CO), the stroke volume (SV) and pulmonary blood volume (PBV) were almost proportional to the bodyweight. Specific results were: DV 120 ml kg-1, CO 136 ml kg-1, SV 1.11 ml kg-1, PBV 6.9 ml kg-1. The pulmonary transit time varied between 1.0 and 3.6 seconds. Clinical applicability of the method, including visual inspection of camera images and quantitative analysis of a time-activity curve of the lung, was demonstrated for one dog with an aortic stenosis and another with a left-to-right shunt.