The purpose of the present experiments was to study whether the uraemic state causes any change in intrinsic cardiac function in the rat. Hearts of animals were studied 24 hr after bilateral nephrectomy (acute uraemia) or two weeks after 5/6 nephrectomy (chronic uraemia). Ventricular function curves were performed in an isolated working rat heart apparatus. Hearts from animals with acute uraemia performed the same as hearts from sham controls. Hearts from animals with chronic uraemia had enhanced performance as evidenced by increased cardiac output, peak and mean left ventricular systolic pressure, maximum rate of left ventricular pressure rise, and external work. Coronary flow, myocardial oxygen consumption, and external efficiency were also greater in hearts of rats with chronic uraemia. These studies demonstrate that short-term chronic uraemia is not associated with depressed intrinsic cardiac performance. On the contrary, isolated hearts from chronic uraemic rats, while performing external work, demonstrate enhanced dynamic and metabolic responses to increasing filling pressures.