Two studies were designed to investigate the time course development of enlarged plasma volume and cholesterol pool size in copper (Cu)-deficient rats as well as influence of Cu deficiency on the lipid composition of lipoproteins. Rats were randomly assigned to three dietary Cu treatments (deficient, marginal, and adequate) in the Study I and two dietary Cu treatments (deficient and adequate) in Study II. Enlargement of plasma volume and cholesterol pool size were established prior to the increase in plasma cholesterol concentration. Cu concentration was decreased, whereas iron and zinc concentrations were increased in the organs of Cu-deficient and Cu-marginal rats. The plasma pool size of VLDL triglyceride was elevated 6-fold, protein and phospholipid were unaltered, and cholesterol was reduced 36%. The plasma pool size of lipid and protein components of HDL and LDL fractions were markedly elevated in Cu-deficient rats.