Pial microcirculation was observed in cats during acutely induced blood pressure increase using the window technique. Measurements of arteriolar diameter changes in situations of preserved and altered cerebrovascular autoregulation were made, when diffuse vasoconstriction and sausage-like dilation, respectively, occurred. Comparison of percentage changes of narrow segments between sausage-like dilated arterioles and diffusely constricted (= autoregulating) vessels gave evidence that a failure of autoregulation in this situation is not associated with vasospasm but with dilation. Individual narrow segments did not constrict more--but on an average even rather less--than diffusely constricted vessels. Evans blue extravasation occurred only with dilation, never with vascular constriction. From these data it is concluded that extravasation due to acute blood pressure increase is not of ischemic nature, but a consequence of high filtration pressure due to arteriolar and venous congestion.