Background: The Rosemary extract (RE) possesses various antioxidant, cytoprotective and cognition-improving bioactivities. In this study, we postulated which doses of RE have a more effect on the hippocampus of middle-aged rats. Methods: In this experimental study, thirty-two middle-aged male Wistar rats were fed by different doses (50,100 and 200 mg/kg/day) of RE (containing 40 carnosic acid) or distilled water for 12 weeks. The effects of different RE doses on learning and spatial memory scores, hippocampal neuronal survival, antioxidant enzymes and lipid peroxidation amount were evaluated by one and two way analysis of variance (ANOVA). Results: It seemed that RE (100mg/kg) could recover the spatial memory retrieval score (p< 0.05). The amount of activity of SOD, GPx and CAT enzymes in the hippocampus of animals of the RE (100mg/kg) group showed a significant increase compared to the normal group (p< 0.01), (p< 0.01) and (p< 0.05), respectively. Also, the amount of activity of GPx in the RE (50 mg/kg) group of animals showed a significant increase compared to the normal group (p< 0.05). No significant difference was found between the groups in the MDA level. Conclusion: The results revealed that rosemary extract (40 carnosic acid) may improve the memory score and oxidative stress activity in middle aged rats in a dose dependent manner, especially in 100mg/kg
