A method and device for long term monitoring of the blood level of various blood constituents. The monitoring is accomplished by capillary filtration and collection of an ultrafiltrate of blood. Filtration of the ultrafiltrate occurs through a filter composed of hollow fibers which are implanted within an interstitial body space in fluid communication with blood capillaries. The ultrafiltrate is drawn through the filter and into an ultrafiltrate collector which extends externally of the body. The collector provides a vacuum for withdrawing the ultrafiltrate and holds discrete liquid samples of the ultrafiltrate. The method and device is particularly suited for long term monitoring of blood glucose levels in diabetic patients or other blood chemical constituents in other patients.