The antidiabetic effect of Parkinsonia aculeata water soluble fraction (WSF) made of aerial parts of the plant (leaves and flowers) was investigated in alloxan diabetic rats. Its effect was compared with that of insulin (positive control). The physico-metabolic parameters measured were: body weight, food and liquid intake, urinary volume, hepatic glycogen, serum glucose, total cholesterol, HDL-cholesterol, triglycerides, urinary glucose and urea, and the weight of epididymal adipose tissue, liver, kidneys and the skeletal muscles (soleus and extensor digitorum longus). Oral administration of WSF (125 or 250 mg/kg) for 16 days exhibited a significant reduction in serum and urinary glucose, urinary urea, total cholesterol, HDL-cholesterol and triglycerides in alloxan diabetic rats. An improvement of hepatic glycogen, a decrease of liquid and food intake, and a significantly positive actions in the weight of skeletal muscles (soleus and extensor digitorum longus) and kidneys were also observed, but just diabetic group treated with WSF at a dose of 125 mg/kg showed significant reduction in urinary volume, body weight, an improvement of epididymal adipose tissue and a positive action in liver weight. The effects of WSF on the physico-metabolic parameters was comparable to those observed in diabetic insulin treated group. The results of this work suggest that P. aculeate may have new clinical significant choice in diabetes mellitus illness, and could explain the basis for its traditional use to manage diabetes-related complications by rural community of northeast of Brazil.