Three long-term, randomised trials examining antihypertensive treatment with either diuretics or beta-blockers are in progress. On the basis of these, the following conclusions may be drawn regarding metabolic side-effects. Diuretics decrease whereas beta-blockers increase serum potassium. Neither drug has any obvious influence on total body potassium. Serum urate increases on both drugs but more so in those taking diuretics. Serum triglycerides increase while total serum cholesterol is unchanged in both treatments. It is still a matter of controversy whether there is a decrease in glucose tolerance and an increase in the incidence of diabetes in patients on diuretics. The possible negative influence of metabolic side-effects on morbidity and mortality may limit the beneficial effects of blood pressure reduction, especially in patients with low pressures before treatment. Thus, it seems crucial to minimise theses side-effects. Since they are dose-related for both diuretics and beta-blockers, the use of low doses and, if needed, combination treatment is preferable.