The effect of NPY on behavior and food intake of food-satiated rats was examined under three different food availability conditions. Food was available during times when rats normally do not eat under either a fixed-ratio or fixed-interval reinforcement schedule, or it was freely available in the bottom of the cage (FF). Forty responses were required for each 45-mg food pellet under the ratio schedule (FR 40) and for the first response to occur 15 s after the previous reinforcement under the interval schedule (FI 15"). NPY (5 micrograms) significantly increased food intake under all conditions and increased food-reinforced responses under the FR and FI schedules. NPY's effect on food intake was greatest when food was freely available and least for rats working under the schedule requiring the most effort (FR 40). Food intake peaked after 3 days under repeated daily administration of NPY. Under free food access and under the fixed-interval schedule, eating and/or responding occurred almost immediately following the onset of the initial 4-h session under NPY. However, during the first session following NPY administration under the FR, rats emitted few responses during the first 2 h of the session. The onset of robust responding under the FR schedule began earlier with each successive daily administration of NPY. These data show NPY substantially increases food-maintained behavior and is a potent inducer of food intake even under conditions where considerable effort is required to obtain food. Further, the conditions under which food is made available can dramatically alter NPY's effect on the temporal pattern of food-maintained responding, feeding, and latency to eat.