The principal objective is the identification of risk factors for the development of intra-operative hypotension or intra-operative hypertension and for post-operative cardiac, renal, and cerebrovascular complications (the presumed result of such fluctuations) among essential hypertensives undergoing elective intra-peritoneal, peripheral vascular or intra-thoracic, but non-cardiac, surgery. The methodology: The study is a prospective cohort in which 200 hypertensive patients who do not have diabetes mellitus, aortic stenosis, congestive heart failure or pulmonary disease with bronchospasm, will be evaluated pre-operatively, monitored intra-operatively, and followed post-operatively. The pre-operative evaluation will include an assessment of the clinical severity of the hypertension, the extent of target organ damage, the co-existence of ischemic heart disease (and other conditions), the plasma volume, the peripheral renin, the functional capacity and the pattern and range of the blood pressure (and pulse) response to sleep, normal activity and stress, including isotonic exercise. Intra-operatively, the patients will be monitored with an Avionics indirect 'pressurometer', which records pulse and ecg as well as blood pressure. Post-operatively, the patients will be followed daily with clinical examination, ecgs, creatinines and CPK isoenzymes. The long term objective is the secondary prevention of the almost 11% incidence of major post-operative complications (MI, CVA, renal failure) which occur in such hypertensives, presumably as a result of intra-operative fluctuations in blood pressure. Strategies for preventing hypotension, however, may be quite different than those designed to prevent hypertension; in fact, a strategy designed to prevent hypertension could be deleterious to a patient at greater risk for hypotension. By identifying specific risk factors for hypotension and those for hypertension, the study will provide the basis for future trials of specific techniques of secondary prevention in each risk category. By providing greater hemodynamic stability among those prone to hypertension or hypotension, such techniques would minimize post-operative complications.