Alterations in activity of kaolin-activated (contact-activated, CA) and immunoreactive (IR) prekallikrein were studied in blood plasma of healthy persons and of patients with hypertension under conditions of rest and of dose-dependent physical loading. CA prekallikrein was dissimilarly altered depending on the type of hemodynamic structure of the pressure reaction (arterial pressure): content of CA prekallikrein was increased in hyperkinetic type and--decreased in hypokinetic type. Content of IR prekallikrein, measured by means of radial immunodiffusion using monospecific antiserum, was similar in healthy persons and in the patients at rest and after physical exercises. It was increased slightly but did not correlate with CA prekallikrein alterations. As content of CA- and IR-prekallikreins did not correlate in the disease dynamics and after physical exercises, functional impairments may cause activation of blood plasma prekallikrein in hypertension. A procedure for isolation of highly purified preparation of kallikrein, production of monospecific antiserum to prekallikrein are described. Immunochemical studies and quantitative estimation of IR-prekallikrein were carried out.