Forty weanling male Sprague rats were randomly assigned to two dietary treatments, copper-deficient (9.0 mumol/kg diet) and copper-adequate (102.2 mumol/kg diet). After 7 wk of treatment, reductions in body weight and hematocrit, and an increase in relative heart weight, were observed in the copper-deficient rats. Plasma VLDL, LDL and HDL were isolated by ultracentrifugation and Sepharose column chromatography. In copper-deficient rats, the percent composition of protein was reduced by one-half, and triglyceride was increased by 1.6- and 2.7-fold in LDL and VLDL fractions, respectively. In VLDL, the marked increase in triglyceride was compensated by at least a 75% decrease in percent composition of cholesterol and phospholipids as a result of copper deficiency. No treatment difference in percent composition of HDL components was detected. When the data were expressed as the amount present in the vascular pool corrected for body weight, the plasma pool size of protein, triglyceride, phospholipid and cholesterol of LDL and HDL were increased twofold or more by copper deficiency. In VLDL, a sixfold increase in triglyceride, a 36% reduction in cholesterol, and no change in phospholipid and protein pool size were observed in copper-deficient rats. These data suggest that copper deficiency may enlarge the particle size but not particle number of VLDL, increase both particle size and number of LDL, and elevate particle number but not size of HDL.