The levels of connective tissue constituents (hexosamine, hexuronic acid, hexose and hydroxyproline) and the composition of isolated mucopolysaccharide-peptide complexes were determined in some organs and tissue of male and female fowl treated and not treated with estradiol-17beta. Most of the changes occurred in the male. Estrogen administration brought statistically significant increases in the contents of hexosamine and hexuronic acid in the cartilage of the male and of hexose in the spleen of both males and females. It also increased the hydroxyproline in the testis and in the cartilage of the male. Estrogen decreased the content of hexosamine in the combs of both males and females, and in the wattle of the female, of hexuronic acid, hexose and hydroxyproline in the liver of the male, and hydroxyproline in the breast muscle of the male. Hexosamine, sialic acid, lipid and protein contents of blood sera of estrogenized birds were substantially higher than that of the controls. Gas liquid chromatography of the lipids of the tissues indicated that estrogen administration brought about an increase in the proportion of the unsaturated fraction to the total fatty acid content.