The microvascular effects of the potassium channel opener, BRL 38227 (the active enantiomer of cromakalim), were investigated in conscious spontaneously hypertensive rats (SHR) provided with a chronic dorsal microcirculatory chamber. BRL 38227 decreased blood pressure and increased heart rate in a dose-dependent manner. It dilated arterioles of different sizes, with the most pronounced effect on the smallest precapillary arterioles. Venule diameters were not significantly affected by BRL 38227. The data show a preferential sensitivity of small precapillary arterioles for potassium channel opening.