This series of studies used a weakly trained (20% methyl anthranilate) version of the passive avoidance learning task in the 1-day-old chick to investigate memory facilitation effects by diphenylhydantoin (DPH). The results indicated that the pairing of the weak training experience with DPH results in facilitation of memory that can be observed from 40 min following training with the weak training experience. The results from a biochemical experiment indicated that DPH facilitates the activity of Na(+)/K(+)-ATPase at the majority of times sampled in a large percentage of the sections of the chick brain. The most marked level of elevation in the activity of the enzyme was observed at the 20-min time point following weak training in the section of the chick brain, which contained several memory relevant neuroanatomical loci. This represents a 68% increase in the activity of the enzyme in those areas considered to be crucial to the processing of memory in the paradigm at a time predicted by previous investigation to be crucial in the development of the intermediate-term memory stage of memory. The results of this series of studies support the notion that Na(+)/K(+)-ATPase plays an important role in memory processing following passive avoidance training in the 1-day-old chick.