Two experiments were conducted to specify the effects of the preexposure of a taste or an odor stimulus upon the compound (taste-odor) aversion conditioning, in which the preexposed stimulus forms a part (T1 ·O1) or in which it forms no part (T2 ·O2). In two-bottle testings, the comparisons were made between following pairs : T1 vs, T2, T1 · 1 vs, T2 ·O2 and O1 vs. O2. In Experiment 1, a preexposure of the taste component (T1) tended to restrain aversion formation to the stimulus (T1), whereas it did not restrain that to the compound one (T1 · O1) as well as that to the odor component (O1) (Figs 1&2). On the other hand, in Experiment 2, a preexposure of the odor component (O1) tended to restrain aversion formation not only to the stimulus (O1) but also restrains that to the compound CS (T1 ·O1), and also to the taste component (T1) (Figs. 3&4).