► Tamoxifen, a PKC inhibitor and antiestrogenic drug, blocked methylphenidate-induced hyperlocomotion. ► Medroxyprogesterone and clomiphene, antiestrogenic drugs, did not block methylphenidate-induced hyperlocomotion. ► These results indicated that the antimanic-like effect of tamoxifen is related to its PKC activity. Tamoxifen, a protein kinase C (PKC) inhibitor and antiestrogenic drug, has clinical antimanic effects and blocks psychostimulant-induced hyperlocomotion. Medroxyprogesterone (MPA), which has antiestrogenic effects, also exerts some clinical benefits in female manic patients and partially blocks amphetamine-induced hyperlocomotion, indicating that the antiestrogenic effect of tamoxifen could contribute to its antimanic effect. The present study evaluated the effect of acute and chronic (21 day) treatment of two antiestrogenic drugs, MPA and clomiphene (an estrogenic receptor antagonist), on methylphenidate (MPH, 5.0 mg/kg)-induced hyperlocomotion in mice, an animal model of mania. Acute and chronic tamoxifen administration was used as a positive control. Acute and chronic tamoxifen (1.0 mg/kg) administration blocked MPH-induced hyperlocomotion. Acute and chronic MPA (acute: 3.0 or 6.0 mg/kg; chronic: 3.0 mg/kg) and clomiphene (acute: 1.5 or 3.0 mg/kg; chronic: 1.5 mg/kg) treatment did not alter MPH-induced hyperlocomotion. These results indicate that tamoxifen exerts antimanic-like effects, and reduced estrogenic activity does not have antimanic-like effects in this psychostimulant-induced hyperlocomotion model. Therefore, the antiestrogenic effect of tamoxifen likely does not contribute to its antimanic effect, which may instead be related to its effect on PKC activity. Therefore, PKC inhibition may be associated with the antimanic effect of mood stabilizers.