We studied histologically the changes and distribution patterns of glycosaminoglycan molecular species during the separation of the lens vesicle in the mouse. Embryos were obtained by sacrificing pregnant mice of the Jcl: ICR strain on day 10.5 and 11 of pregnancy. Serial frontal sections were stained with hematoxylin-eosin and a sensitized high iron diamine method. To identify glycosaminoglycan molecular species in tissues, enzyme digestion (double digestions with chondroitinase B and testicular hyaluronidase) and chemical modification (nitrous acid treatment) were performed in combination with the sensitized high iron diamine method. Before separation of the lens vesicle, the glycosaminoglycan molecular species, identified in the basement membrane of the presumed corneal epithelium and intercellular matrices between the presumed corneal epithelium and lens vesicle, were chondroitin sulfate A/C and B, and those in the lens capsule were chondroitin sulfate A/C. After separation of the lens vesicle, heparan sulfate emerged in the basement membrane of the presumed corneal epithelium and intercellular matrices between the presumed corneal epithelium and lens vesicle. These results are thus taken to indicate that the changes and distribution patterns of glycosaminoglycan molecular species play an important role during separation of the lens vesicle.