Several haemodynamic variables were measured in sixteen well-trained athletes at rest and during muscular exercise before and after two months of a daily oral dose of 15 mg of metandienone, an anabolic steroid (17alpha-methyl-17beta-hydroxy-1,4-androstadien-3-one). Their training programmes were the same as they had been for at least three years before the study. During metandienone treatment the total blood volume increased by about 15%, and the heart rate decreased significantly both at rest and during exercise. The resting values of cardiac index and stroke index did not differ significantly, while during muscular exercise cardiac index showed a tendency to increase, and stroke index increased by 20% during the treatment period. When treatment was discontinued, the haemodynamic values returned to pre-medication levels, and when a repetition of the treatment was carried out, exactly the same changes were seen. The resting forearm blood flow did not differ significantly during the treatment, while the peak flow during post-ischaemic hyperaemia increased by 27%. The pulmonary blood volume, peripheral resistance and mean arterial pressure did not differ significantly after metandienone treatment. The mean body weight increased significantly during the treatment period.