The effect of the GABAB agonist baclofen on cocaine self-administration in the rat was investigated. In the first experiment, rats trained to self-administer i.v. cocaine (1.5 mg/kg/inj) on a progressive ratio (PR) schedule were pretreated with various doses of baclofen (1.25, 2.5, or 5.0 mg/kg). Baclofen produced a dose-dependent decrease in the break points. In the second experiment, baclofen (2.5 mg/kg) was found to decrease significantly break points across a series of unit injection doses of cocaine (0.18, 0.37, 0.75, 1.5 mg/kg/inj). Baclofen produced only modest effects on food-reinforced responding even at the largest dose tested (5.0 mg/kg). These data suggest that baclofen may produce a specific attenuation of cocaine reinforcement. Baclofen produced no significant change in the rate of i.v. cocaine intake on a fixed ratio (FR 1) schedule. These data support a number of recent observations that rate of drug intake may be an insensitive measure of changes in the motivation to self-administer cocaine.