Copper deficiency results in defective elastin and collagen maturation in most tissues. A close relationship also exists between these components and proteoglycans in connective tissue. In an effort to obtain information on the nature of proteoglycans in copper deficiency, the composition of glycosaminoglycans in lungs from copper-deficient (1 micrograms/g of diet) or -supplemented (25 micrograms/g diet) chicks was studied. The total glycosaminoglycan concentration in copper-deficient chick lungs did not differ from that in control chick lungs. However, variations in individual glycosaminoglycan concentrations between lungs from copper-deficient and -supplemented chicks were observed. Heparan sulfate and dermatan sulfate concentrations were lower in copper-deficient chick lungs than in controls. The glycosaminoglycans from lungs of copper-deficient chicks also had lower molecular weights than glycosaminoglycans from lungs of control birds.