Effect of prazosin (0.1 and 1.0 mg/kg b.w. i.v.) on the cardiovascular system was investigated in six anaesthetized Beagles (Na-pentobarbital 35 mg/kg i.p.) using the thermodilution and catheter methods. The following parameters were measured: a) mean arterial blood pressure, b) heart rate, c) total peripheral resistance, d) cardiac output, e) cardiac work, f) stroke volume and g) cardiac contractility. Results can be summarized as follows: 1. Prazosin in dose of 0.1 mg/kg i.v. caused a slight drop in arterial blood pressure; prazosin administered in dose of 1.0 mg/kg i.v. decreased blood pressure significantly. 2. Following doses of 0.1 and 1.0 mg/kg i.v. of prazosin, heart rate was significantly increased in each of six dogs. 3. When prazosin was given i.v. (0.1 and 1.0 mg/kg), an initial increase in cardiac output occurred. Later, a reduction in cardiac index below the control values could be observed. 4. The contractility parameters of the left ventricle--dp/dtmax and VCE--were increased after i.v. administration of 0.1 and 1.0 mg/kg b.w. of prazosin.