It is difficult to determine the precise role of the N-methyl-D-aspartate (NMDA) receptor system in the reinforcing effects of cocaine since uncompetitive NMDA receptor antagonists alter cocaine self-administration in different ways, depending on the antagonist examined and the behavior being measured. To increase understanding of the role of the NMDA system in cocaine's reinforcing effects, this study measured the effects of the competitive NMDA receptor antagonist, LY235959 [(-)-6-phosphonomethyl-deca-hydroisoquinoline-3-carboxylic acid], in rats that self-administered cocaine under both fixed ratio (FR) 1 and progressive ratio (PR) schedules of reinforcement. Rats were trained to self-administer cocaine (0.33 mg/infusion) under an FR1 schedule of reinforcement. Thereafter, the effects of pretreatment with LY235959, or the uncompetitive antagonists dextromethorphan and dizocilpine, were examined. The number of infusions earned during the first 10 min of responding under the FR1 schedule was analyzed separately. When rats responded for 0.33 mg/infusion cocaine under an FR1 schedule of reinforcement, 3 mg/kg LY235959 decreased cocaine self-administration only during the first 10 min of the responding. This effect was dose and time dependent and blocked by the competitive NMDA receptor agonist, NMDA. LY235959 (3 mg/kg) decreased total responding for cocaine only when the self-administered dose of cocaine was small (0.02-0.04 mg/infusion) or when responding was reinforced under the PR schedule. In contrast, dizocilpine decreased responding under the FR1 schedule but increased responding under the PR schedule. These data suggest that LY235959 decreased the reinforcing effectiveness of cocaine, a finding reported with systemically administered NMDA receptor antagonists other than dizocilpine.