The dynamic elastic properties and vascular reactions of human blood vessels in vivo are studied by means of the photoplethysmographic method. The measurement of the mechanical properties of the blood vessel walls is based on measurement of blood volume changes resulting from changes in external pressure. The blood volume changes are mon itored by measuring the opacity changes in vascular bed, in this case, the distal phalanx of the finger. The elastic properties thus measured reflect some average property of a whole vascular bed. The technique is simple, noninvasive, and does not cause any damage or discomfort to the subject. In healthy young subjects, blood vessel elasticity was found to be constant in the range between the systolic and diastolic pressure. However the volume-pressure curves showed a hysteresis loop, i.e., the curves obtained during the increase of intravascular pressure differed from those obtained when pressure was decreased. The elasticity of the vascular walls of patients suffering from peripheral vascular diseases was found to be significantly different; their volume- pressure curves were nonlinear and had a much wider hysteresis loop (indicating a lower critical opening pressure), and their vascular reactions to changes in pressure were different. Possible uses of the method for diagnosing and evaluating arterio sclerotic and other vascular diseases and for studying vascular changes and reactions under conditions such as shock, heart failure, external abnormal pressures, and various drugs, are discussed.