The effects of d-amphetamine (0.32 to 5.6 mg/kg) and diazepam (0.10 to 5.6 mg/kg) on defecation were examined in two groups of rats. One group was exposed to a fixed-time 60-s (FT 60-s) schedule of food delivery and the other group was exposed to massed-food sessions. During vehicle control sessions, rats exposed to the FT 60-s schedule excreted a significantly higher number of fecal boli than rats exposed to massed-food sessions. d-Amphetamine, at doses above 0.56 mg/kg, significantly reduced defecation (boli produced) in both groups, although the magnitude of the drug's effect was larger in the group exposed to the FT 60-s schedule. For both groups, diazepam only produced a significant decrease in defecation at the highest dose (5.6 mg/kg). These results appear to be inconsistent with interpretations of adjunctive behavior that emphasize arousal or emotion as mechanisms.