We have devised a system for analyzing the temporal coherence of capillary pressure (CP) and capillary red blood cell velocity (CBV) variations in humans. The system is designed to measure human blood pressure in skin capillaries by direct cannulation while simultaneously measuring red blood cell velocity in the same capillary by video microscopy. The servo-nulling pressure measurement system allows the dynamic recording of capillary pressure with a flat frequency response of about 12 Hz. Computerized data acquisition is synchronized with the frame code of a U-matic video recorder, which records the capillaroscopic picture for later computerized off-line analysis of capillary red blood cell velocity. Measurements of simultaneously recorded CP and CBV in healthy volunteers show synchronized pulsation. Minimal pulsatile variations in CP values result in marked variations of CBV. In addition, minor fluctuations (3-4 cycles per minute) in CP are accompanied by marked changes in CBV. This system may help provide information about microvascular pathophysiology in skin diseases before and after treatment.