The purpose of the study was to investigate the effects of Opuntia megacantha leaf extracts on blood glucose concentrations and kidney function in normal and streptozotocin (STZ)-diabetic rats. STZ-diabetic and non-diabetic rats were orally administered extracts of O. megacantha leaves (20 mg/100 g body weight) daily for 5 weeks and respective control rats were administered normal saline (0.1 ml/100 mg body weight). Urine volume, urinary outputs of Na+, K+ and creatinine were monitored daily over the 5-week period. Plasma concentrations of Na+, K+, urea and creatinine and the glomerular filtration rate (GFR) as assessed by creatinine clearance were determined after 5 weeks. Plasma glucose concentrations in STZ-diabetic and non-diabetic rats were reduced by the administration of leaf extracts of O. megacantha. However, leaf extracts increased urinary Na+ output in STZ-diabetic and non-diabetic rats, concomitantly with a reduction in plasma concentration of the ion. O. megacantha leaf extracts significantly increased plasma creatinine and urea concentrations in non-diabetic and STZ-diabetic rats. Administration of the leaf extract was also associated with an increased GFR in STZ-diabetic rats (from 1.8 +/- 0.3 ml/min to 2.8 +/- 0.3 ml/min, n = 8) although the rate was unaltered in non-diabetic rats. The results suggest that leaf extracts of O. megacantha not only reduce blood glucose levels, but may be toxic to the kidney as shown by the elevation in plasma urea and creatinine concentrations and the reduction of plasma Na+ concentration.