Information from animal studies has demonstrated the harmful effects of zinc supplementation on copper transport. Although some studies have been carried out in humans, the effect on humans has not been as well documented as the effect on animals. Forty-seven healthy volunteers (26 women and 21 men) took part in a double-blind cross-over trial which lasted for 12 weeks. The subjects were asked to take 50 mg of elemental zinc (as 220 mg zinc sulphate) or placebo, three times a day for six weeks. Venous blood was collected for the analysis of plasma levels of copper and zinc. Symptoms, which included headaches, abdominal cramps, nausea, loss of appetite and vomiting, were registered from 84% of women and 18% of men. Six female volunteers discontinued the trial; five owing to gastric irritation and one owing to consistent headaches. Plasma zinc levels rose significantly in both men and women, the increase being 36% and 57%, respectively. Plasma copper levels did not change significantly. Our study suggests that the gastric discomfort that is associated with zinc supplementation may be related to body weight as symptoms were reported from the lower-weight volunteers. Our study also shows no detrimental effect of 150 mg of zinc a day on plasma copper levels in healthy volunteers over a period of six weeks.