1. Centella asiatica, an Indian medicinal plant, has been described as possessing central nervous system activity, such as improving intelligence. In addition, we have demonstrated that C. asiatica has cognitive-enhancing and anti-oxidant properties in normal rats. Oxidative stress or an impaired endogenous anti-oxidant mechanism is an important factor that has been implicated in Alzheimer's disease (AD) and cognitive deficits seen in the elderly. 2. Intracerebroventricular (i.c.v.) streptozotocin (STZ) in rats has been likened to sporadic AD in humans and the cognitive impairment is associated with free radical generation in this model. Therefore, in the present study, the effect of an aqueous extract of C. asiatica (100, 200 and 300 mg/kg for 21 days) was evaluated in i.c.v. STZ-induced cognitive impairment and oxidative stress in rats. 3. Male Wistar rats were injected with STZ (3 mg/kg, i.c.v.) bilaterally on the days 1 and 3. Cognitive behaviour was assessed using passive avoidance and elevated plus-maze paradigms on the days 13, 14 and 21. Rats were killed on the day 21 for estimation of oxidative stress parameters (malondialdehyde (MDA), glutathione, superoxide dismutase and catalase) in the whole brain upon completion of the behavioural task. 4. Rats treated with C. asiatica showed a dose-dependent increase in cognitive behaviour in both paradigms. A significant decrease in MDA and an increase in glutathione and catalase levels were observed only in rats treated with 200 and 300 mg/kg C. asiatica. 5. The present findings indicate that an aqueous extract of C. asiatica is effective in preventing the cognitive deficits, as well as the oxidative stress, caused by i.c.v. STZ in rats.