Gastrulation was examined in Xenopus embryos injected with various polysaccharides into the blastocoel cavity. The progression of gastrulation was assessed by observing pigmentation and yolk plug size in vegetative view embryos. In heparin- or dextran-sulphate-injected embryos, gastrulation was significantly retarded. This was further confirmed in tissue sections of embryos. In contrast, no such retardation was found in embryos injected with hyaluronic acid or chondroitin sulphate. A quantitative analysis showed that the extent of retardation in heparin- or dextran-sulphate-injected embryos was dose-dependent and that, after the initial retardation of up to 2-3 h, gastrulation progressed at a similar rate to controls. At the time when untreated sibling embryos hatched, embryos injected with heparin or dextran sulphate showed abnormalities in their external appearance and swimming behavior in a dose-dependent manner. When these embryos were examined histologically or immunohistochemically using tissue-specific monoclonal antibodies, it was found that central nervous system (CNS), especially the brain and eye structures, were most severely damaged. The extent of damage was again dose-dependent. In contrast, neural-crest-derived melanophores were abundant even in aneural larvae. No such change was found in embryos injected with hyaluronic acid or chondroitin sulphate.