Objective. Mallotus oppositifolius (Geiseler) Müll. Arg. (Euphorbiaceae) is folklorically used to “treat” diabetic conditions in some parts of Nigeria therefore the study, to investigate the extract of the leaves for activities on hyperglycaemia, lipid peroxidation, and increased cholesterol levels in vivo in alloxan diabetic rats as well as its potential antioxidant activity in vitro. Methods. Albino rats (240–280 g) were given an injection of 120 mg/kg body weight, i.p. of alloxan monohydrate. After 8 days, diabetic animals with elevated fasting blood glucose levels (>9 mmol/L) were considered and selected for the study. Results. Oral treatment with the extract administered every 12 h by gavage at doses of 100, 200, and 400 mg/kg of the extract to the test rats, for 14 days, resulted in a significant dose-dependent decrease in blood glucose levels from 12.82 ± 1.02 mmol/dL to 4.92 ± 2.01 mmol/dL at the highest dose of 400 mg/kg compared to the control drug and glibenclamide as well as attendant significant decline in diabetic rats employed in the study. Conclusion. The extract also showed in vitro concentration-dependent antioxidant activity following the 1,1-diphenyl-2-picryl-hydrazyl (DPPH) and ferric reducing assays. Findings further suggest the presence of active antidiabetic and antioxidant principles in M. oppositifolius leaves.