The purpose of this study was to investigate in more detail the characteristics of the age-related extension of the retrograde amnesia gradient previously demonstrated in a passive avoidance task . In Experiment 1, it was found that while 2-3 month old mice were susceptible to the amnesic effects of anisomycin (ANI) only when given prior to 15 min post-training, memory of 14-16 month old mice was susceptible to disruption when ANI was given as late as 20 min post-training, and retention of 17-20 month old mice was impaired when ANI was injected even as late as 30 min after training. Experiment 2 examined whether the age-related change in susceptibility to the effects of ANI could be ameliorated by chronic pretreatment with a choline-enriched diet. Results showed that ANI injected 20 min after training did not induce amnesia in choline treated mice (14.5 month old), but did induce amnesia when injected 15 min post training. Subsequent assay of choline acetyltransferase (ChAT) and tyrosine hydroxylase (TH) activity showed that choline treatment significantly reduced ChAT activity but did not affect TH activity. It appears that dietary choline treatment can render new long-term memories less susceptible to disruption following training.