The usefulness of end-systolic measures of ventricular function was compared with that of standard contractility indexes in conscious dogs. End-systolic relations between left ventricular pressure and volume and between pressure and wall thickness were analyzed in dogs previously instrumented with ultrasonic crystals. Progressive angiotensin infusions were used to generate computer-averaged pressure-volume and pressure-wall thickness loops. Both relations were linear in every study and highly reproducible. With low and high dose dobutamine, the end-systolic pressure-volume relations were significantly displaced, with increased slope and inconsistent changes in intercept. This relation was more useful than the ejection fraction for detecting contractility increases at different afterloads, but it showed no advantage over maximal left ventricular dP/dt at all ranges of preload and afterload. The end-systolic pressure-volume relations were insensitive for detecting mild decreases in inotropic state produced by propranolol, and maximal dP/dt was superior for detecting such mild acutely reduced contractility. The end-systolic pressure-wall thickness relations showed displacement with dobutamine, although slope and intercept changes were not significant; these relations did not detect mild decreases in contractility produced by propranolol. It is concluded that the end-systolic pressure-volume relation and a simplified end-systolic measure using pressure and wall thickness provide sensitive, load-independent and reproducible approaches for defining acute increases in left ventricular contractility in conscious animals. Maximal dP/dt was equally effective for defining these increases in contractility and more sensitive for detecting slight acute decreases in contractility.