Three paired-associate learning (PAL) experiments were conducted with Japanese learning materials and Japanese Ss. Backward associations were tested at various stages of a-b learning. With noun-pairs, backward associations were superior to forward early in learning, but in later stages they were 85%-92% as strong as forward associations. With nonsense syllable-and familiarized nonsense syllable-pairs, backward associations, always significantly inferior to forward, reached the maximum when two-thirds of the a-b list was learned, and later approached an asymptote approximately 50% as strong as forward associations. Facilitating effects of familiarization on backward associations were limited to early stages of learning. A given directionality of training seems to be one of the determinants of PAL. Availability of terms may not sufficiently explain the whole process of associative learning. Asymmetry may be the general rule, and synimetry may be its special case.