The methanol-dichloromethane leaf extract (MDE) of Ocimum basilicum L. (Lamiaceae)was subjected to biological activity-guided studies using antihyperglycemic effect in alloxan diabetic rats as activity-guide. The hypoglycemic and oral glucose tolerance effects were also evaluated in normal rats. The results showed that the extract and the fractions caused a significant (P<0.05) reduction in blood glucose level of alloxan diabetic rats. The level of reduction in blood glucose at 8 h was of the order of magnitude: MDE (Methanol-dichloromethane extract; 48.70%) > N-HF (N-Hexane fraction; 44.74%) > MF (Methanol fraction; 42.28%) > DF (Dichloromethane fraction; 31.88%). In normal rats, however, the extract did not reduce blood glucose level at doses tested but suppressed postprandial blood glucose rise after glucose load. Acute toxicity studies on the extract revealed an oral LD50> 5 g/kg in mice. Preliminary phytochemical studies showed that the extract and fractions tested positive to carbohydrate, glycosides, reducing sugars, resins, saponins, steroids, and terpenoids. These findings suggest that constituents of leaves of O.basilicummay possess antihyperglycemic properties and lower blood glucose only in diabetic but not in normoglycemic condition. The antihyperglycemic activity may not be attributed to a single constituent.