Equilibrium gated radionuclide ventriculography is one of the most important fields of work in nuclear cardiology. The in vivo labelled method utilizing 99mTc and red cells labelled with pyrophosphate is one of the most widely used. In the present work determination of the optimum 99mTc activity was studied. A sample of 30 patients was divided into five groups which received various amounts of radioactivity: 303 MBq, 444 MBq, 617 MBq, 803 MBq and 1020 MBq. The image quality of each study was assessed qualitatively and by using a method based on the construction of an image quality discriminant function. The number of counts in regions of interest in the heart, liver, lung and spleen, and the background, were processed as variables. The best results for the selected variables were obtained with 803 MBq.