We investigated in rats whether giving a sweet substance following a food (a 'dessert') would reinforce a preference for that food. The sweet substance had the reverse effect--rats preference for a flavor decreased if the flavor was given in a food preceding a sweet substance (saccharin or sucrose). If the substances were given in the reverse order, so the sweet substance preceded the food, the rats preferred a sweet substance that had been followed by food to one that had not been followed by food. We suggested two hypotheses to account for the data. Perhaps the sweet substance elicits a negative reaction that is unpleasant unless food is given. Thus, food following a sweet substance is reinforcing, while a sweet substance following food is not. A not incompatible alternative is that anticipatory contrast or comparison effects are involved. Assuming the sweet substance is preferred to the non-sweet, following food by a sweet substance could make the food less valued (anticipatory negative contrast), whereas following a sweet substance by food could make the sweet substance more valued (anticipatory positive contrast).