Hyperhomocysteinemia and -amyloid (A) overproduction are critical etiological and pathological factors in Alzheimer disease, respectively; however, the intrinsic link between them is still missing. Here, we found that A levels increased and amyloid precursor protein (APP) levels simultaneously decreased in hyperhomocysteinemic rats after a 2-week induction by vena caudalis injection of homocysteine. Concurrently, both the mRNA and protein levels of presenilin-1, a component of ␥-secretase, were elevated, whereas the expression levels of -secretase and presenilin-2 were not altered. We also observed that levels of phosphorylated APP at threonine-668, a crucial site facilitating the amyloidogenic cleavage of APP, increased in rats with hyperhomocysteinemia, although the phosphorylation per se did not increase the binding capacity of pT668-APP to the secretases. The enhanced phosphorylation of APP in these rats was not relevant to either c-Jun N-terminal kinase or cyclin-dependent kinase-5. A prominent spatial memory deficit was detected in rats with hyperhomocysteinemia. Simultaneous supplementation of folate and vitamin-B12 attenuated the hyperhomocysteinemia-induced abnormal processing of APP and improved memory. Our data revealed that hyperhomocysteinemia could increase A production through the enhanced expression of ␥-secretase and APP phosphorylation , causing memory deficits that could be rescued by folate and vitamin-B12 treatment in these rats. It is suggested that hyperhomocysteinemia may serve as an upstream factor for increased A production as seen in patients with Alzheimer disease.