Background: General anesthesia and increasing age are two main risk factors for postoperative cognitive dysfunction (POCD). Effective agents for the prevention or treatment of POCD are urgently needed. L-655,708, an inverse agonist of alpha 5 subunit-containing gamma-aminobutyric acid subtype A (alpha 5GABA(A)) receptors, can prevent anesthesia-induced memory deficits in young animals. However, there is a lack of evidence of its efficacy in old animals. Methodology: Young (3-to 5-month-old) and old (18-to 20-month-old) mice were given an inhalation of 1.33% isoflurane for 1 hour and their associative memory was evaluated 24 hours after anesthesia using fear-conditioning tests (FCTs). To evaluate the effect of L-655,708, mice received intraperitoneal injections of L-655,708 (0.7 mg/kg) or vehicle 30 minutes before anesthesia. Results: Old mice exhibited impaired memory and lower hippocampal alpha 5GABA(A) levels than young mice under physiological conditions. Pre-injections of L-655,708 significantly alleviated isoflurane-induced memory decline in young mice, but not in old mice. Conclusions: L-655,708 is not as effective for the prevention of POCD in old mice as it is in young mice. The use of inverse agonists of alpha 5GABA(A) in preventing POCD in old patients should be carefully considered.