Corn oil is reported to elicit a conditioned place preference (CPP) in a CPP test in mice. To further investigate a reinforcing effect of corn oil, we studied whether the corn oil acts as a reinforcer under a progressive ratio (PR) schedule in the operant task. BALB/c mice were trained to lever press for sucrose and corn oil. After reaching a stable break-point for 100% corn oil, the PR test was conducted for various concentrations of corn oil (0%-100%). The reinforcing effect of corn oil was increased in a concentration-dependent manner under the PR schedule. A mineral oil and 0.3% xanthan gum as vehicles did not show any reinforcing effect in the PR test, suggesting that oily and viscous texture was not related to the reinforcing property of corn oil. The break-point for corn oil was attenuated by pretreatment with (-)-sulpiride, a D(2) antagonist, in the PR test. On the other hand, SCH23390, a D(1) antagonist, did not influence the break-point. Furthermore, the pretreatment with (-)-sulpiride or SCH23390 did not influence the intake of corn oil in a one-bottle test for 30 min, suggesting that the dopaminergic system is involved in the reinforcing effect but not the consumption of corn oil in mice. In conclusion, operant response to corn oil is concentration-dependently enhanced under the PR schedule. This reinforcing effect of corn oil is at least partly mediated through the dopaminergic systems via D(2) receptors.