We investigated the effects of exercise training on the amount of aortic collagen and systolic blood pressure in spontaneously hypertensive rats (SHR). Ten-week old SHR were trained either by forced treadmill running (26.8 m X min-1 -1 h X day-1, five times a week, 0% incline) or by voluntary running in revolving wheels (7,800 m X day-1 at peak) for 8 weeks. Succinate dehydrogenase (SDH) activity measured as a marker of an endurance training effect was 13% higher (P less than 0.01) in the soleus of forced-exercised animals than in that of sedentary ones. (6.56 +/- 0.17 mumol X g-1 X min-1; mean +/- SEM), whereas SDH activity in that of voluntarily-exercised group was found to be at the same level as in sedentary animals. The systolic blood pressure after training increased by 26.4 in sedentary, 21.1 in voluntarily-exercised, and 33.9 mm Hg in forced-exercised rats, when compared with the value of each group at the beginning of the training program. A significant difference was observed in the increment of blood pressure only between the voluntarily- and forced-exercised groups (P less than 0.05). The amount of aortic collagen in voluntarily-trained rats (96.5 +/- 2.0 mg X g tissue-1, 39.8 +/- 0.7 mg X 100 mg protein-1) was significantly less than that in forced-trained rats (P less than 0.05). These results suggest that voluntary, mild exercise training may be more effective in the reduction of collagen accumulation in the aorta associated with the suppression of blood pressure increase than forced, vigorous exercise training in SHR.