Measurement of venous function in vivo is inherently difficult. In this study we used the Hilbert transform to examine the dynamic relationships between venous pressure and cardiac output in rainbow trout whose blood volume was continuously increased and decreased by ramp infusion and withdrawal (I/W). The dorsal aorta and ductus Cuvier were cannulated percutaneously and connected to pressure transducers; a flow probe was placed around the ventral aorta. Whole blood from a donor was then I/W via the dorsal aortic cannula at a rate of 10% of the estimated blood volume per min, the duration of I/W was varied from 40, 60, 80, 90, 120, 230, 240, 260, 300, 340 seconds. Compliance (blood vol/venous pressure) was 2.8 ±0.2 ml/µmHg^-1/kg^-1 (N=25 measurements; 6 fish with closed pericardium) and 2.8 ±0.3 ml/µmHg^-1kg^-1 (N=19 measurements, 4 fish with open pericardium). Compliance was positively correlated with the duration of I/W, indicative of cardiovascular reflex responses at longer I/W durations. In trout with closed pericardium, cardiac output (CO) followed venous pressure oscillations with an average time lag of 4.2 ±1.0 seconds (N=9); heart rate (HR) was inversely correlated with CO. These studies show that cardiac output is entrained by modulation of venous pressure, not by heart rate. Thus although trout have a rigid pericardium, venous pressure (vis-a-tergo), not cardiac suction (vis-a-fronte), appears to be the primary determinant of CO. Estimation of venous compliance by ramp-modulation of venous pressure is faster and less traumatic than classical capacitance measurements and appears applicable to a variety of vertebrate species, as does the Hilbert transform, which permits analysis of signals with disparate frequencies.