The field work of the baseline or prevalence phase of the Gubbio Study on the Epidemiology of Hypertension was conducted between March 1983 and December 1985. The population sample included all individuals age 5 and over residing in the town of Gubbio. The survey involved a standardized comprehensive examination focussed on traits previously shown to be related to hypertensive and/or other adult cardiovascular diseases, and on other traits of current concern as possible risk factors. This latter category included measurement of red blood cell electrolyte concentrations and fluxes, particularly sodium-stimulated lithium countertransport. The study was planned, organized, and conducted by the Merck, Sharp and Dohme Center for Epidemiologic Research in cooperation with Gubbio civic and medical leaders, and with laboratory facilities in Gubbio, Naples, and Rome.