We have studied the interrelation of left ventricle and arterial system in the anesthetized open-thorax cat. The ventricle was characterized by its pump function graph, relating mean ventricular pressure (Plv) and mean aortic flow (F). The pump function graph was determined by means of an artificial periphery and on a beat-to-beat basis. The periphery was characterized by relating mean aortic pressure (Pao) and mean flow. Mean aortic and mean left ventricular pressure could be related over a wide range of values by a proportionality factor Pao = a . Plv. In a series of five separate experiments a value of a = 1.72 +/- 0.14 (mean +/- SD) was found. This simplified relation allows direct comparison of apparent source resistance (i.e., slope of pump function graph), (Rs), and peripheral resistance (Rp). It was also found experimentally that total external power (w) could be calculated from mean aortic pressure and mean flow as well as from mean left ventricular pressure and mean flow (thus from the pump function graph) by w = c . Pao . F = c . a . Plv . F. The value of c = 1.16 +/- 0.12 (mean +/- SD, n = 4). Maximum external power was predicted for Rp/Rs = Pao/Plv = a. In six different cats Rp/Rs ratio in the working point (i.e., mean left ventricular pressure and mean flow when the normal periphery loaded the heart) was found to be Rp/Rs = 2.63 +/- 0.92. This value could not be shown to differ from that in the point where maximal external power was found, i.e., Rp/Rs = 1.81 +/- 0.08 (n = 6).