We report results in rats pretreated with (+/-)-zacopride (0.03 mg/kg, IP), ICS 205-930 (0.1 mg/kg, IP), and MDL 72222 (1.0 mg/kg, IP) 15 min before challenge with (-)-cocaine (10.0 mg/kg, IP). At a dose of 10 micrograms/kg, zacopride significantly inhibited (approximately 50%) cocaine-induced locomotion. We also investigated whether or not 5-hydroxytryptamine3 (5-HT3) antagonists block the cocaine binding site on the dopamine transporter and/or affect the ability of dopamine to regulate this binding site. In well-washed striatal membranes, neither zacopride nor ICS 205-930 (10(-9)-10(-5) M) inhibited [3H]2 beta-carbomethoxy-3 beta-(4-fluorophenyl)tropane ([3H]WIN 35,428) (0.3 nM) binding. Furthermore, neither of these compounds affected the ability of dopamine to block WIN 35,428 binding. To determine if 5-HT is required for the 5-HT3 antagonist effect, we examined the interaction between cocaine and zacopride in rats pretreated with p-chlorophenylalanine (PCPA) (3 days x 100 mg/kg/day). PCPA pretreatment shifted the cocaine dose-response curve to the right and blocked the ability of zacopride to reverse cocaine-induced activity.