The relationships between body mass index (BMI) and age, alcohol consumption, 24-hr urinary electrolyte excretion, and BP were studied in 588 subjects from three German centers participating in Intersalt, a highly standardized, previously reported protocol. Men and women aged 20-59 were sampled in Bernried, FRG; Cottbus, GDR; and Heidelberg, FRG. The subjects from the three centers did not differ in BMI, level of education, physical activity, cigarette- or alcohol-consumption patterns, or urinary Cl excretion. Mean Na excretion was 167, 147, and 172 mmol/24 hr in Bernried, Cottbus, and Heidelberg, while mean K excretion was 72, 55, and 73 mmol/24 hr, respectively. The excretion of these electrolytes was significantly lower in Cottbus than in Bernried or Heidelberg. BMI increased progressively in men with age; in women BMI plateaued until the 5th decade, after which it increased to equal that of men. In individual centers, the excretion of electrolytes was correlated with BMI. Sodium and chloride excretion were highly correlated. The data from each individual center were fitted to a multiple regression model. Age, BMI, sex, and alcohol consumption entered the model.