A serial recall experiment under same- and cross-modality conditions was carried out according to Hebb's (1961) paradigm. Subjects memorized strings of nine digits in 18 trials, which were divided into early trials and late ones. Under the same-modality condition the strings used in the 18 trials were presented through the same modality, i.e., visual or auditory, whereas under the cross-modality condition the strings used in the early and late trials were presented through two different modalities. A repeated string and unrepeated strings were presented 3 and 6 times respectively in both early and late trials. The results showed that (1) repetitive learning in the same-modality occurred for both visual and auditory; (2) there was a non-additivity of cross-modal repetitive learning from auditory to visual, whereas there was an additivity of cross-modal repetitive learning from visual to auditory. We inferred that the visual inputs received both visual coding and auditory coding through an audio-transformation so as to be separately memorized in visual and auditory memories, whereas the auditory inputs were memorized in the auditory memory through the auditory coding alone.