Myocardial performance was evaluated in rhesus monkeys after endotoxin shock, and the responses to fluid loading with colloid measured in both anesthetized control and experimental groups. Minute work and cardiac output (CO) were decreased in five monkeys after six hours endotoxin to levels significantly below control values. Infusing colloid to a mean left ventricular end-diastolic pressure (LVEDP) of 12-15 mm Hg increased both CO and minute work significantly but they remained one-half that of the control group of four primates after fluid loading. Improved cardiac performance persisted after infusion through a 30-minute recovery stage when LVEDP returned to normal. Simultaneous pulmonary arterial wedge (PAW) pressures showed some correlation with LVEDP reading up to six mm Hg, but above that level the PAW underestimated the LVEDP by 3-6 mm Hg. Microscopic study showed that fluid loading produced comparable pulmonary edema in both groups, but endotoxin produced ultrastructural capillary lesions.