We assessed the changes in cardiovascular function in humans caused by RO 13-6438, a new drug that enhances cardiac performance, by means of noninvasive methods which included measurement of systolic time intervals and electrical impedance cardiography. Twelve healthy male volunteers received RO 13-6438 doses of 10 and 20 mg intravenously and 20, 40, and 60 mg orally according to a double blind, randomized, crossover, placebo-controlled design. A dose-dependent distinct enhancement in cardiac performance was seen. This was attributed to positive inotropism (shortening of heart rate-corrected electromechanical systole) and to a vasodilating action (decline of diastolic blood pressure and total peripheral resistance). In addition, the drug increased heart rate slightly. The cardiac effects were detectable for 6 h. To reach the average equivalent inotropic response over 6 h, the oral dose was approximately 1.8-fold of the i.v.; this indicated a high bioavailability of RO 13-6438. Transient color vision disturbances were reported mainly following the intravenous administration. The properties of RO 13-6438 suggests that it may be useful for treatment of heart failure.