The effects of dl-cathinone (dl-CAT), d-amphetamine (d-A), and cocaine (COC) on conditioned shock avoidance responding and their interactions with haloperidol and methysergide on this behavior were studied in male Wistar rats. All three stimulants produced significant increases in intertrial interval (ITI) responding and in the number of avoidance responses and a decrease in avoidance latencies. These actions were antagonized by pretreatment with haloperidol (0.07 and 0.15 mg/kg, IP). Pretreatment with methysergide (1.0 and 2.0 mg/kg, IP) increased the effects of all three stimulants on ITI responding, but not on the other two parameters. These results suggest that the effects of these stimulants on avoidance responding may be mediated by dopaminergic systems. In addition, these stimulant-induced changes on ITI responding probably also involve actions on serotonergic systems.