The clinical, morphologic, histochemical, and biochemical features of GM1-gangliosidosis in two canine models, English Springer Spaniel (ESS) and Portuguese Water Dog (PWD), have been compared. The disease onset, its clinical course, and survival period of the affected dogs were similar in both models. Skeletal dysplasia was noted radiographically at 2 months of age, whereas at 4 1/2 months of age there was progressive neurologic impairment. However, dwarfism and coarse facial features were seen only in ESS. Both models had similar deficiency in activity of lysosomal beta-galactosidase, but possessed a normal protein activator for GM1-beta-galactosidase. Both models stored GM1-ganglioside, asialo-GM1, and oligosaccharides in brain. Furthermore, only the PWD stored glycoproteins containing polylactosaminoglycans in visceral organs, and neither model stored them in the brain. Morphologically, both models demonstrated similar storage material in multiple tissues and cell types. The ultrastructure of the storage material was cell-type specific and identical in both models. However, some differences in the lectin staining pattern were noted. Our clinical, biochemical, and histochemical findings indicate that PWD and ESS may represent two different mutations of the beta-galactosidase gene. Moreover, the authors conclude that it is difficult, and inappropriate, to apply the human classification of GM1-gangliosidosis (i.e. infantile, juvenile, and adult forms) to these canine models.