Male Wistar rats were separated into 4 groups: group 1, thiamine sufficient diet (control); group 2, thiamine sufficient diet with intraperitoneal administration of furosemide (20 mg . kg-1 of body weight); group 3, thiamine deficient diet; group 4, thiamine deficient diet within tra-peritoneal administration of furosemide. After 4 weeks, the rats were killed and the thiamine levels and activity of transketolase were assayed. Thiamine concentration and transketolase activity were significantly decreased and thiamine pyrophosphate effect was significantly increased in the blood, and various tissues in group 2 and 4 compared with group 1 and 3, respectively. The intraperitoneal administration of various concentrations of furosemide (20 mg, 10 mg, and 2 mg . kg-1 of body weight) resulted in a significant increase in urinary thiamine excretion. Thus, it is assumed that long-term administration of furosemide could induce a thiamine deficiency.