The vasoactive properties of potassium were assessed in the microcirculation of the hamster cremaster muscle and the muscular and epithelial portions of the hamster cheek pouch. Tissues were transilluminated and suffused with a physiological salt solution whose potassium concentration varied from 0 to 20 mM. Vessel diameters were measured and normalized as a percent of the control diameter (+/- SE) observed during exposure to 4.7 mM K+. Altering the potassium concentration in the suffusion solution caused a transient vascular response. The peak changes in the vascular diameter of the arterioles supplying striated muscle varied directly with the suffusion solution potassium concentration from a minimum of 78 +/- 3% in 0 mM K+ to 155 +/- 15% in 15 mM K+. Vascular diameter increases were sustained for the full 5-minute test period only in 15 mM K+. In the epithelial portions of the cheek pouch, only the constrictor component of the potassium response was observed. The data indicate that potassium is sufficiently potent to participate in initiating functional hyperemia in striated muscle and might cause as much as a 6.3-fold increase in flow. Functional hyperemias exceeding approximately 3 minutes cannot be due to potassium ion, since the dilation induced by this agent is transient.