In previous experiments using the fat substitute sucrose polyester (SPE, or olestra), no compensatory response was observed on day 2 after experimental manipulations, which reduced the percentage of energy from fat to approximately equal to 30% from 40% on day 1. In the present study a more severe manipulation was made; the amount of energy from fat was reduced from 32% to 20% to determine whether such a reduction would provoke any physiologic or behavioral response. Subjects came to the unit for two, 2-d test sessions. Intake on day 1 was fixed: subjects were given meals containing either control fat (11319 kJ, 32% of energy as fat) or SPE (9561 kJ, 20% of energy as fat). On day 2, intake was ad libitum. On day 1 subjects rated themselves as more hungry while consuming the fat-substituted meals than when consuming the control meals and they disclosed greater hunger in the end-of-day questionnaires. The effect of the manipulation was carried over into day 2. By the end of day 2, subjects had compensated for 74% of the energy (fat) deficit caused by the previous day's manipulation. These results differ from those obtained when fat was reduced from 40% to 30% of energy; this more severe reduction reveals that a reduction in fat of this size can lead to a biobehavioral response. Together, these data suggest that people could change their diet to meet dietary guidelines; however, if a more severe reduction is attempted, adherence may be made more difficult by the strength of the compensatory response.