Introduction. Catharanthus roseus (L.) is used in some countries to treat diabetes. The aim of this study was to evaluate the hypoglycemic activity of extracts from the flower, leaf, stem, and root in normal and alloxan-induced diabetic mice. Methods. Roots, leaves, flowers, and stems were separated to obtain organic and aqueous extracts. The blood glucose lowering activity of these extracts was determinate in healthy and alloxan-induced (75 mg/Kg) diabetic mice, after intraperitoneal administration (250 mg/Kg body weight). Blood samples were obtained and blood glucose levels were analyzed employing a glucometer. The data were statistically compared by ANOVA. The most active extract was fractioned. Phytochemical screen and chromatographic studies were also done. Results. The aqueous extracts from C. roseus reduced the blood glucose of both healthy and diabetic mice. The aqueous stem extract (250 mg/Kg) and its alkaloid-free fraction (300 mg/Kg) significantly (P < 0.05) reduced blood glucose in diabetic mice by 52.90 and 51.21%. Their hypoglycemic activity was comparable to tolbutamide (58.1%, P < 0.05). Conclusions. The best hypoglycemic activity was presented for the aqueous extracts and by alkaloid-free stem aqueous fraction. This fraction is formed by three polyphenols compounds.