The amnesic effects of cycloheximide (CYC) on habits of different ages were investigated in a food-motivated, discrimination-reversal task. Groups of C57BL/6J mice were injected 30 min before training or immediately, 3 days, 6 days, or 9 days after training. Retention was tested 24 hr after CYC treatment. The usual amnesic effect of CYC occurred in animals injected before training. No amnesia was apparent in groups injected immediately, 3 days, or 9 days after training. However, a reliable and reproducible amnesia occurred in the group injected 6 days after training. This amnesia could be reversed by pretest treatment with a monoamine oxidase inhibitor, pheniprazine. Pheniprazine, given 7 days after training, also enhanced retrieval of memory in saline-injected mice.Although amnesia can sometimes result from posttraining injections of cycloheximide (CYC) or acetoxycycloheximide (AXM) (e.g., Geller, Robustelli, Barondes, Cohen, & Jarvik, 1969;Quartermain & McEwen, 1970), the majority of experiments employing multiple-trial tasks failed to demonstrate amnesia when CYC or AXM is given after training (e.g., Barondes & Cohen, 1967;Daniels, 1972;Flexner & Flexner, 1966;Squire & Barondes, 1972). These findings have led to a general acceptance of the notion that protein synthesis must be inhibited while learning is taking place in order for an impairment in memory to be subsequently demonstrated.The ineffectiveness of posttraining CYC treatment in multiple-trial tasks is most commonly accounted for by reference to the consolidation hypothesis. Briefly, this hypothesis assumes that by the time the training trials are terminated, or shortly thereafter, the memory trace has consolidated into a storage stage (either intermediate or long-term memory), which is no longer susceptible to disruption by protein-synthesis inhibition (McGaugh, 1966(McGaugh, , 1968). An alter-This study was supported by Grant Foundation, Incorporated. We thank G. B. Whitfleld of the Upjohn Company forsupplying the cycloheximide.Requests for reprints should be sent to David