Crayfish (29.8 g) maintained at 25° C were examined with regard to their cardiovascular pressure-flow relationships. Pulsatile arterial hemolymph flows were simultaneously recorded in the three major arteries leading from the heart. Hemolymph pressures were measured throughout the cardiovascular system. Tail movements were also monitored. Flows and pressures were found to vary little during quiet periods. Arterial hemolymph flows were 1.3 mL · min⁻¹ in the anterior aorta, 5.2 mL · min⁻¹ in the sternal artery, and 0.8 mL · min⁻¹ in the posterior aorta. Mean heartbeat frequency was 125.6 ± 5.2 beats · min⁻¹, with a stroke volume of 2.0 mL · kg⁻¹ · beat⁻¹ and a cardiac output of 252 mL · kg⁻¹ · min⁻¹ (summation of arterial flows). Systolic intracardiac pressure was 9.5 mmHg, with a mean arterial pressure of 7.5 mmHg. Infrabranchial and pericardial sinus pressures (the pressure gradient allowing hemolymph to flow through the complex branchial vasculature) were 3.2 mmHg and 2.5 mmHg, respectively. During perio...