Mammalian sialidases (NEU1, NEU2, NEU3 and NEU4) that remove sialic acids from glycoconjugates have been implicated in diverse cellular functions. Human sialidases are involved in the development of various disease states such as cancer, diabetes and arteriosclerosis. Unregulated acidic sialidase NEU1 activity is associated with the pathogenesis of lysosomal storage disorder (LSD) sialidosis, abnormal immune responses and cancer progression. Obesity is closely related to several chronic diseases such as diabetes, cardiovascular diseases, hyperlipidemia or hypertension that are associated with metabolic syndrome. We examined fluctuations in mRNA levels and sialidase activities of NEU1 in two strains of obese and diabetic mice to assess the involvement of NEU1 in obesity. The activity of NEU1 was preferentially higher in epididymal fat and lower in the livers of two strains of obese and diabetic mice. Fluctuations in NEU1 activity might be associated with the pathological status of these tissues in obesity.