Serum copper and zinc concentrations (mumol/L) were measured in nonfasting subjects aged 25-64 y participating in two independent population surveys in Northern Ireland. In 1983-84, copper in 1144 males was 17.2 +/- 3.1 (mean +/- SD) and zinc was 12.1 +/- 1.7 (SD). Copper in 1055 females, neither pregnant nor taking estrogens or progestogens, was 19.0 +/- 3.9 and zinc was 11.6 +/- 1.4. In 1986-87 copper in 1142 males was 17.9 +/- 3.3 and zinc was 13.2 +/- 2.1. Copper in 1034 females was 20.1 +/- 3.9 and zinc was 12.7 +/- 2.0. Zinc but not copper concentrations decreased from early morning to late afternoon; both were unaffected by reported postprandial time. There was a positive relationship between copper and age for both sexes but zinc showed only a slight upward trend with age. A positive relationship between copper and the aggregation of classical risk factors for coronary heart disease was demonstrated.