The time course of shifts induced by physical training in the resting cardiovascular parameters was investigated in swim-trained albino rats. Measurements were performed weekly, both during a 14-weeks training period and 5 weeks after cessation of regular swimming. Cardiac output (dye dilution) blood pressure (electromanometry) and heart rate (ECG) were measured under intraperitoneal urethan anesthesia. In comparison with non-exercising controls, rats at the end of the training period displayed lower heart rate, smaller stroke volume and cardiac output, and a higher peripheral resistance. The early phase of regular training, however, was characterized by an elevated cardiac output, heart rate and stroke volume, and by a decreased peripheral resistance. After discontinuation of regular training, cardiac output was elevated as well, but this was brought about only by the increased stroke volume, because resting heart rate was still lower than in the controls. Blood pressure did not change during the whole experimental period. Considering that similar shifts have been reported in humans, the changes in the resting cardiovascular status may be responsible for the similar complaints and symptoms often observed in undertrained athletes or in athletes who had interrupted high intensity regular training.