In cats with implanted ultrasonic transducers, interrelationships between phasic oscillations of the blood flow in posterior v. cava due to the chest sucking function and changes of intrapleural pressure, were studied. Linear dependence of shifts of these parameters was only observed in an increased pressure and respective drop in venous blood flow. The dependence was absent in drop of the pressure and increase of venous blood flow. The ratio of the intrapleural pressure shift to magnitude of phasic changes of the blood flow in the posterior v. cava (the pumping coefficient) is suggested for estimation of effect of the chest sucking function upon the venous outflow and for relative estimation of rigidity of the vascular bed's venous portion.