SUMMARY Direct intravital microscopy was used to analyze microclrculatory changes in the exteriorized spinotrapezius muscle of spontaneously hypertensive rats (SHR). The animals were anesthetized with a mixture of chloralose-urethane, and measurements made of pressure, flow, and resistance in vessels ranging in size from 50 to 5 Mm. Tbe vascular changes in SHR were compared with matched Wistar-Kyoto (WKY) strain animals for both young (5-6 weeks old) and mature (12-13 weeks) rats. Distinctive changes in tbe distribution and levels of pressure, flow, and resistance were seen in tbe entire mlcrovascular network during both stages of the syndrome. There was no significant increase in the resistance of the conduit arteries just proximal to tbe muscle proper. Blood pressure in hypertensives was brought down to normal and even below normal at the level of the capillaries and postcapiiiaries irrespective of tbe height of the pressure in the major artery supplying the muscle. The greater drop in pressure across the arteriolar branchings of tbe hypertensives was seen as early as at 5-6 weeks of age; this difference Is much more striking in 12-13 week-old mature hypertensives. The reduction in pressure was proportionately greatest in the region of the smallest (10-15 /im) precapillaries of hypertensives. Resistance values were below normal in the confluent capillaries and postcapiiiaries in both young and mature hypertensives. Volumetric flow, which was marginally higher throughout the arteriolar branchings, fell below normal on the postcapillary side. Since an increased resistance developed at an early age (5-6 weeks) in all of the microvessels on the precapillary side, tbe suggestion is advanced that hypertension is associated with a generalized effect on tbe muscular arterioles below 30 urn, an effect that becomes more pronounced with time and at 12-13 weeks begins to involve larger sized arterioles (