We propose a novel method to dissociate incentive motivation from memory and motor processes in instrumental performance. Components of a multiple fixed-ratio schedule of food reinforcement were adjusted to envelop the range of response requirements that maintained lever pressing by rats. We sought to manipulate motivation for food rewards with acute administrations of various doses (0-10 mg/kg) of fluoxetine, a 5-HT reuptake inhibitor that reduces food intake. A quantitative model of fixed-ratio performance derived from Killeen's (1994) Mathematical Principles of Reinforcement (MPR) provided an adequate account of data from individual rats. Decreases in response rate resulting from fluoxetine were reflected in changes in estimates of activation, indexed by MPR parameter a; estimates of working memory capacity and lever pressing duration were not systematically affected. These results support the use of MPR parameter a to index incentive motivation using multiple fixed-ratio schedules that are adjusted to individual performance.