1. The haemodynamic effects of a novel cardiotonic drug, levosimendan, which has both calcium-sensitizing and phosphodiesterase III (PDE III) inhibitory properties, were studied in conscious dogs in which heart failure had been induced by prolonged cardiac pacing in the presence of aortic constriction. These effects were compared with those in sham-operated dogs with essentially normal cardiac function. 2. Eighteen mongrel dogs were instrumented for the measurement of left ventricular pressure (LVSP, LVEDP) and contractile function (dP/dt; dP/dt/P). In twelve dogs a balloon catheter, positioned in the thoracic aorta, was inflated producing an approximate 60% reduction in effective aortic diameter. Twenty min later rapid ventricular pacing (240 beats mean-1) was commenced and maintained for 48 h by means of a bipolar pacing electrode introduced into the right ventricle. This electrode served also for recording changes in the endocardial electrogram in the absence of pacing. Six of these dogs were used to evaluate the haemodynamic changes of pacing-induced heart failure; a further six of these dogs the haemodynamic changes elicited by levosimendan under these conditions. Six sham-operated dogs (group 2) served as controls. 3. In six dogs (group 1) the haemodynamic alterations were assessed after the development of heart failure. In the presence of aortic constriction, 48 h continuous rapid cardiac pacing resulted in a marked deterioration in left ventricular function which remained stable for at least 48 h after cessation of pacing. Thus, there was a marked reduction in LVSP (15%), +dP/dtmax (35%), -dP/dtmax (36%) and also in dP/dt/P (29%), whereas LVEDP was increased considerably (from 6.4 +/- 1.4 to 20.0 +/- 2.2 mmHg). A marked elevation occurred in endocardial ST-segment (138%), lasting for 20 min.4. Levosimendan was administered intravenously in doses of 0.005, 0.01 and 0.03 micromol kg-1 to 2 groups of conscious dogs. In the sham-operated dogs (group 2), only the higher dose (0.03 micromol kg-1)produced significant increases in LVSP (19%), + dP/dtmax (37%), and in dP/dt/P (32%). In dogs with heart failure (group 3) doses of 0.005, 0.01 and 0.03 micromol kg-1 levosimendan resulted in an improve mentin +dP/dtmax (26%, 38% and 49%), -dP/dtmax (20%, 25% and 38%) and in dP/dt/P (19%, 34%and 50%) and reduction in the elevated LVEDP (from 20 =/- 2.2 mmHg to 16 +/- 1.0, 10 +/- 1.3 and 9 +/- 1.0 mmHg, respectively).5. Levosimendan proved to be a potent cardiotonic drug at the doses used, and was approximately three times more effective under conditions of impaired left ventricular function than in normal hearts.