We have developed a method for reducing cardiac output in a controlled stepwise fashion using awake, intact, unsedated lambs. The method involves placing a balloon-tipped (Foley) catheter into the right atrium from a jugular vein isolated by a small neck incision. Systemic venous return and cardiac output are limited by balloon inflation. With each balloon inflation the animal reaches a new and stable cardiac output, which allows the measure of steady-state hemodynamic and metabolic variables. We have been able to decrease cardiac output to as low as 20% of the resting cardiac output and maintain a stable preparation. The reductions in cardiac output are quickly reversible by balloon deflation. Animal survival allows repeated study. We present data from five lambs studied between 26 and 36 days of age. Alterations in O2 consumption, O2 transport, O2 extraction, blood pressure, and arterial lactate concentration are examined in response to decrements in systemic blood flow and are consistent with changes seen in response to a reduction of cardiac output by other methods.