- Morphine has paradoxical effects in learning experiments. The drug can serve as a reinforcer in several situations; yet rats avoid tastes paired with morphine, much as they avoid tastes paired with an emetic drug such as lithium chloride (LiCl). The results of the present experiment indicate that, in contrast with LiCl-induced taste avoidance, the strength of morphine-induced avoidance is nonmonotonically related to the duration of training. Although taste avoidances produced by both drugs are readily established, the morphine-induced avoidance (unlike the LiCl-induced avoidance) weakens with continued flavor-drug pairings. These results, together with prior findings, suggest that there are distinctive features of morphine-induced taste avoidance.