The changes in the resistance of the perfused ventral rat-tail artery resulting from exposure of the tissue to ouabain-containing and/or K+-free physiological salt solution were studied. In each case, there was an increase in the vascular resistance which was not sustained. The response of the arterial wall to the above stimuli was abolished in the absence of external Ca2+. In contrast to the delayed response of the wall to either ouabain-containing or K+-free solution, an almost instantaneous rise in the resistance was observed if the two stimuli were combined, though the rate of loss of the tissue K+ was not accelerated significantly under these experimental conditions. The tension developed in K+-free solution was relieved almost instantaneously upon readmittance of external potassium.