The present study was designed to evaluate the effect of Mexican poppy on memory of unstressed and stressed Swiss young albino mice. Mexican poppy (500mg/kg i.p) and piracetam (200 mg/kg i.p.), were administered for 21 successive days to separate groups of unstressed and stressed mice. The nootropic activity was evaluated using elevated plus maze and Hebbs Williams Maze. Brain acetylcholinesterase (AChE), brain nitrite and plasma corticosterone levels were also estimated. unpredictable chronic mild stress was produced by using different stressors. Mexican poppy (500mg/kg i.p) and piracetam (200 mg/kg i.p.), significantly showed memory enhancing activity in both unstressed and stressed mice. Mexican poppy significantly reduced brain AChE activity and brain nitrite levels in both unstressed and stressed mice. Mexican poppy (500mg/kg i.p) significantly reversed scopolamine-induced amnesia in unstressed and stressed mice. 7-Nitroindazole [a neuronal nitric oxide synthase (NOS) inhibitor] and aminoguanidine (an inducible NOS inhibitor) significantly enhanced memory improving activity and brain nitrite decreasing effect of Mexican poppy in unstressed and stressed mice respectively. Plasma corticosterone levels were significantly decreased by Mexican poppy (500mg/kg i.p) in stressed mice as compared to its control. Thus, Mexican poppy showed memory enhancing activity in unstressed mice probably by decreasing brain AChE activity and by inhibition of neuronal NOS. The memory enhancing activity of Mexican poppy instressed mice might be due to decrease in brain AChE activity, inhibition of inducible NOS and by decreasing the elevated plasma corticosterone levels.