Hemodynamic measurements were made in 29 patients undergoing 31 episodes of genitourinary instrumentation. The conditions of two patients were studied twice during separate surgical episodes. Bacteremia developed with 21 procedures, gram-negative organisms in eight procedures, gram-positive organisms in ten, and both gram-negative and gram-positive organisms in three procedures. Three patients manifested clinical shock. Immediate hemodynamic effects of gram-negative bacteremia are a decrease in systemic vascular resistance and central venous pressure and an increase in cardiac output. Clinical shock is an exaggeration of these changes, but the fall in central venous pressure prevents the cardiac output from compensating for the fall in systemic vascular resistance. Patients with gram-positive bacteremia and abacteremic patients undergoing similar instrumentations do not demonstrate comparable hemodynamic changes.