The fresh and dried rhizome of Zingiber officinale Roscoe (commonly known as ginger) is widely used in traditional medicine. We have studied the effect of the juice of Z. officinale (4 mL kg(-1), p.o. daily) for 6 weeks on streptozotocin (STZ)-induced type I diabetic rats with particular reference to the involvement of serotonin (5-hydroxytryptamine; 5-HT) receptors in glycaemic control. In normoglycaemic rats, 5-HT (1mg kg(-1), i.p.) produced hyperglycaemia and hypoinsulinaemia, which was significantly prevented by the juice of Z. officinale. STZ-diabetes produced a significant increase in fasting glucose levels that was associated with a significant decrease in serum insulin levels. Treatment with Z. officinale produced a significant increase in insulin levels and a decrease in fasting glucose levels in diabetic rats. In an oral glucose tolerance test, treatment with Z. officinale was found to decrease significantly the area under the curve of glucose and to increase the area under the curve of insulin in STZ-diabetic rats. Treatment with Z. officinale also caused a decrease in serum cholesterol, serum triglyceride and blood pressure in diabetic rats. Our data suggest a potential antidiabetic activity of the juice of Z. officinale in type I diabetic rats, possibly involving 5-HT receptors.