This is the first of two reports dealing with the reliability of measurements of 24-hour urinary electrolyte excretion and blood pressure and estimates of electrolyte-blood pressure associations in INTERSALT, an international study of the relations of electrolyte excretion and other factors to blood pressure, involving more than 10,000 persons from 52 centers in 32 countries. This first report describes methods for estimating reliability, taking into account age and sex, and provides estimates for several urinary variables, blood pressure, and pulse rate. The second report (Am J Epidemiol 1994; 139:940-51) uses these estimates of reliability and multivariate procedures to correct multiple regression coefficients from regressions of blood pressure on 24-hour urinary sodium and potassium excretion, body mass index, and alcohol intake for "regression dilution bias." Age- and sex-adjusted estimates of reliability were computed from data on 805 INTERSALT participants with repeat measurements. These estimates ranged from 0.37 to 0.40 for 24-hour urinary sodium, from 0.47 to 0.52 for potassium, from 0.32 to 0.36 for the sodium:potassium ratio, from 0.64 to 0.69 for calcium, from 0.59 to 0.65 for creatinine, from 0.49 to 0.57 for urinary volume, from 0.49 to 0.51 for magnesium, from 0.58 to 0.62 for pulse, from 0.69 to 0.74 for systolic blood pressure, and from 0.63 to 0.67 for diastolic blood pressure. In addition, estimates of within- and between-person covariances among electrolytes indicated that about half of the observed covariance for sodium and potassium excretion in a single 24-hour urine collection was due to within-person covariation in excretion.