The dorsal skin flap technique was used to study skeletal muscle microcirculation in conscious 10-12-week-old spontaneously hypertensive rats and normotensive Wistar-Kyoto control rats. Videorecordings were made for off-line analysis of consecutive segments of the vascular bed. Resting diameters were significantly smaller in spontaneously hypertensive rats than in Wistar-Kyoto rats at the first-order (-28%) and second-order arteriolar (-21%) levels. Precapillary third-order and fourth-order arterioles of spontaneously hypertensive rats had normal diameters, whereas postcapillary small venule diameters were slightly larger in spontaneously hypertensive rats. Thirty percent and 41% of the spontaneously hypertensive rat and Wistar-Kyoto rat third-order arteriolar vessels and 63 and 45% of the fourth-order arteriolar vessels exhibited vasomotion. Vasomotion amplitude, but not frequency, was significantly higher in spontaneously hypertensive rats than in Wistar-Kyoto rats. It is concluded that, in the established phase of spontaneous hypertension in the rat, a decreased diameter of large arterioles is the major mechanism underlying the increased vascular resistance in cutaneous skeletal muscle.