1. We compared the effects of two doses (5 and 10 mg) of oral pimobendane (UD-CG 115) on haemodynamics in eight patients suffering from chronic congestive heart failure. The two doses were given according to a randomized cross-over double-blind protocol; haemodynamics and plasma levels of pimobendane and its main metabolite UD-CG 212, were determined 1, 2, 3, 5, 7, 9, 11 and 12 h after each dose. 2. Both doses significantly improved the left and right ventricular functions of these patients, with a peak action 3 h after drug intake and long duration (more than 12 h). A significant dose-effect relationship was observed only for pulmonary wedge pressure and right atrial pressure. Significant correlations were found between UD-CG 212 plasma levels and cardiac index (r = 0.54, P less than 0.05), and pulmonary wedge pressure (r = 0.74, P less than 0.001); no correlation was found between these haemodynamic variables and pimobendane plasma levels. 3. One patient developed a transient drop in blood platelets together with a cutaneous rash, while three others had a transient and mild decrease of thrombocytes. 4. In conclusion, pimobendane improved right and left ventricular functions in severe heart failure. Both doses (5 and 10 mg) were effective. The higher dose induced marked improvement of the haemodynamic variables but the difference between doses was only significant for right atrial and pulmonary wedge pressures.