The cardiodynamic effects of Prostaglandin E1 were studied in sheep with congestive heart failure. Six Merino ewes were equipped surgically with high fidelity micromanometers and left ventricular pressures measured. Heart rate, left ventricular systolic and end-diastolic pressures, [+dP/dt]max and a single-exponential time constant (T) were calculated on a computer from the recorded pressure curves and the Wilcoxon signed rank test used to determine the statistical significance of the differences. Prostaglandin E1 (20 micrograms.kg-1) significantly improved left ventricular contractility and relaxation rate as seen respectively from [+dP/dt]max and the single-exponential time-constant and reduced the afterload conditions under which the heart operated, during various stages of congestive heat failure, without significant tachycardia during the more advanced stages of the disease. The results show that [+dP/dt]max increased between 16% (stage 4) and 38% (stage 1); T shortened between 14% (stage 4) and 25% (stage 1) and left ventricular systolic pressure decreased between 14% (stage 4) and 21% (stage 1) when prostaglandin E1 was administered during four stages of left ventricular pump failure, which ranged from a 20% decrease in [+dP/dt]max (congestive heart failure stage 1) to a 50% decrease in [+dP/dt]max (congestive heart failure stage 4). In contrast to normal animals, prostaglandin E1 caused an increase in left ventricular end-diastolic pressure during the more advanced stages of pump failure. Nevertheless the favourable cardiodynamic changes brought about by prostaglandin E1 administration might contribute, together with afterload reduction, to a general haemodynamic improvement during congestive heart failure.