We applied the principles of cardiac mechanics to study the contraction cycles of postnodal sheep mesenteric lymphatic vessels in an in situ preparation. A segment of intestinal lymphatic was isolated from lymph input and provided with Krebs solution from a reservoir. Pressure-volume relationships were assessed under various transmural pressure conditions using videomicroscopic techniques to measure diameter changes and a miniature catheter pressure transducer to monitor intralymphangion pressure. The contraction cycles were quite variable but, on average, exhibited three phases of systole and three phases of diastole with periods of isovolumetric contraction and relaxation. Elevations of transmural pressure up to 4 cm H2O resulted in significant increases in stroke volume, ejection fraction, pulse pressure, and output/minute but not contraction frequency. Regression analysis of the end systolic (ESPVR) and end diastolic pressure-volume relations (EDPVR) revealed a linear ESPVR (r2 = 0.83 +/- 0.03) and exponential EDPVR (r2 = 0.83 +/- 0.02), a result that is similar to that observed in cardiac contraction cycles. Following a 25% whole blood volume bleed (a stimulus known to enhance lymphatic pumping), significant increases in stroke volume, ejection fraction, and output/minute were observed up to transmural pressures of 4 cm H2O. While an index used to assess cardiac contractility (end systolic elastance-Ees) was not observed to change after the bleed, a shift to the left of the end-systolic pressure-volume relations compared with the sham-bled group (significant shift of regression lines for ESPVR) suggested that hemorrhage exerted a positive inotropic effect on mesenteric lymphatics.