Mice were run in a T-maze for food reward, to investigate possible effects of deprivation level on "overnight decrement," i.e., worse performance on the first trial of a given day than on the last trial of either the previous or the same day. Following an initial training period, each mouse received a series of massed trials on 22 consecutive days. On half of these days, the mice were deprived of food in their home cages; on the other half, they were not deprived. Overnight decrement was substantial only when the mice were not deprived. These results have important implications concerning the comparabiliry of positively reinforcing brain stimulation and more conventional rewards.Overnight decrement, i.e., worse performance on the first trial of a given day than on the lasc trial of eicher the previous or the same day, is one of several effects that in the past have been considered to be relatively unique to behavior reinforced by electrical stimulation of the brain. While such decrement has repeatedly been observed in behavior maintained by brain stimulation, the phenomenon has only infrequently been noted in behavior reinforced with such conventional rewards as food or water (Trowill, Panksepp, & Gandelrnan, 1969;Lenzer, 1972). Thus, overnight decrement, together with various other brainstimulation effects like rapid extinction, poor performance with intermittent reward, strong priming effects, and persistent dominance of a preference for brain stimulation over conventional rewards, has been taken by some researchers as reflective of an inherent characteristic of brain-stimulation itself (Trowill, et al., 1969;Lenzer, 1972).This conclusion, however, has been challenged by a number of findings which suggest that these seemingly unique characteristics simply derive from important differences between the experimental procedures that are rypically used with brain-stimulation reinforcement and those usually employed with conventional rewards. These differences include deprivation level, magnitude and delay of reward, and the complexity of the reinforced behavior (Trowill, et al., 1969;Lenzer, 1972). Thus, by manipulating these factors, researchers have largely dissipated the aura of anomaly from the rapid extinction, poor performance with intermittent reward, strong priming and dominance effects of behavior maintained by brain stimulation (Trowill, et al., 1969;Lenzer, 1972). As yet, however, overnight decrement still awaits investigation as the possible product of specific experimental conditions rather than the type of stimulus employed as a reinforcer.