Oxovanadium(iv) complexes with Schiff bases derived from substituted salicylaldehyde and diamine have been prepared and characterized. 1H NMR and IR spectral data revealed that the symmetrical Schiff base was isolated, and elemental analysis confirmed the purity of all the compounds as formulated. The room-temperature magnetic moments of 1.6–1.8 Bohr magneton for the complexes confirmed that the complexes are VIV complexes, with d1 configuration. Cyclic voltammetry revealed only one quasi-reversible wave for each complex and they all showed redox couples with peak-to-peak separation values (ΔEp) ranging from 76 to 84 mV, indicating a single-step one-electron transfer process. An oral administration of these complexes supplied at a dose of 0.2 mmol kg–1 to streptozotocin-induced diabetic rats elicited a progressive reduction in plasma glucose over 6-h periods. Two of the complexes produced significant and consistent reduction in fasting blood glucose levels over a 6-h monitoring period.