The effects of stepwise reductions of arterial pressure on arteriolar diameter and on vasomotion patterns in the rabbit tenuissimus muscle were investigated in eight New Zealand White rabbits (0.8-1.2 kg) anesthetized with 20% urethane. The vasomotor activity of 13 bifurcations, where first-order terminal arterioles branch from a transverse arteriole, was recorded by video microscopy and related to the pressures in the femoral artery. The arterial pressure was lowered in steps by partial occlusion of the abdominal aorta. Changes of mean diameter due to reduced perfusion pressures were most pronounced in transverse arterioles. Dilation in terminal arterioles at reduced arterial pressures did not exceed resting condition vasomotion peak diameters. Regular vasomotion in the terminal arterioles was intercalated with periods of no vasoactivity which became progressively longer when perfusion pressure was reduced. The oscillation frequency was maintained during the periods with regular vasomotion. Vasomotion ceased in transverse and terminal arterioles at arterial pressures between 50 and 30 mm Hg. During reactive hyperemia vasomotion reappeared after 0.5 to 4 min with the original fundamental frequency. We postulate that vasomotion in terminal arterioles is due to a vascular pacemaker which acts as a local oscillator that can be influenced by perfusion pressure in an on/off-type fashion. The pacemaker oscillation frequency is constant and independent of myogenic factors.