Quantitative measurements of microvascular behavior were carried out by direct microscopy in the mesentery and skeletal muscles of normal and hypertensive rats, and in the skeletal muscle of the cat. Included were measurements of flow pressure in individual segments of the microcirculation from arterioles through the capillaries and into the venules. Variations in the resistance of the terminal arterioles are precapillaries were plotted in response to representative agonists and to pharmacologic blockade. In both the spontaneous hypertensive rat and the DOCA hypertensive rat, the early phase of the syndrome, 3 - 5 weeks after the blood pressure begins to rise above control levels, was associated with a significantly higher flow rate throughout the network in the mesentery and in the spinotrapezius muscle. Subsequently, many of the venules and arterioles show a strong tendency to become coiled and tortuous. There was an increase in the microvascular response to norepinephrine as measured by a change in flow rate in selected terminal arterioles (20 - 30 microns wide) in hypertensive rat tissues of animals with a sustained elevation in blood pressure for from 3 - 10 weeks. BIBLIOGRAPHIC REFERENCES: Zweifach, B. W. and H. H. Lipowsky. Microvascular resistances based on direct measurements of pressure and flow. In: "Cardiovascular System Dynamics." (J. Boon, A. Nordergraaf and J. Raines, eds.), M.I.T. Press, 1977. Hargens, A. R. and Zweifach, B. W. Contractile stimuli in collecting lymph vessels. (In press, Amer. J. Physiol., 1977).