The effect of methylprednisolone on the hydrolysis of adenine nucleotides by rat blood serum enzymes was studied. Adult male Wistar rats were submitted to three different treatments with synthetic steroid methylprednisolone: one dose of 50 mg/kg, i.p. (acute); or oral doses of 6 mg/kg dissolved in drinking water for 15 (sub-chronic) or 30 (chronic) days. Decreased ADP hydrolysis was observed after acute and sub-chronic treatments. Furthermore, ATP, ADP and AMP hydrolysis decreased after chronic treatment. These alterations may constitute one of the mechanisms that mediate the development of some of the side effects associated with corticosteroid use.