The main objective of this investigation is to characterize the changes which occur int he peripheral circulation of rats as a result of the aging process. The parameters to be investigated include alterations in: peripheral vascular resistance, blood flow distribution, microvascular morphology and reactivity, and capillary density and perfusion. An evaluation of blood flow distribution and peripheral vascular resistance will be made using the isotope labelled microsphere technique. Direct in vivo microscopic observation will be used to assess the contributions of specific areas of the microvasculature to the changes which occur with the aging process. A comparison of the parameters described above will be made in adult and senescent rats.