Haloperidol, a dopamine D2 receptor blocker, is a classical neuroleptic drug that elicits extrapyramidal symptoms. Its metabolites include 3-(4-fluorobenzoyl) propionic acid (FBPA) and 4-(4-chlorophenyl)-4-piperidinol (CPHP). Until now, the biological significance of these metabolites has remained largely unknown. Here, we report that the administration of FBPA to mice effected a suppression of locomotor activity and induced catalepsy in a manner similar to that observed with haloperidol, whereas CPHP had no significant effects. Neither of these two metabolites, however, exhibited any ability to bind to the dopamine D2 receptor. FBPA blocked dopamine-induced extracellular signal-regulated kinase 1/2 phosphorylation, and it specifically affected mitogen-activated protein kinase kinase (MEK)1/2 activity in hippocampal HN33 cells. Moreover, FBPA was capable of direct interaction with MEK1/2, and inhibited its activity in vitro. We demonstrated the generation of haloperidol metabolites within haloperidol-treated cells by mass spectrometric analyses. Collectively, our results confirm the biological activity of FBPA, and provide initial clues as to the receptor-independent role of haloperidol.