Tetrabenazine is considered to act in a manner similar to reserpine to reduce the involuntary movements of tardive dyskinesia or Huntington's disease and to improve psychoses. We determined that tetrabenazine also has properties of a dopamine receptor antagonist by testing the ability of tetrabenazine to block the inhibitory effect of dopamine on prolactin secretion from rat anterior pituitary glands in vitro and to displace 3H-spiperone binding to dopamine receptors in the pituitary, corpus striatum, and a rat transplantable prolactin-secreting tumor. Under in vitro conditions, 0.5 to 10 μM tetrabenazine directly blocked dopaminergic inhibition of prolactin secretion. Furthermore, 1 hour after tetrabenazine injection (30 mg/kg intraperitoneally) in vivo, when the serum prolactin had increased from 22 ± 9 to 450 ± 52 ng/ml (p < 0.01), pituitary glands of the treated rats examined in vitro were refractory to dopaminergic inhibition of prolactin release. Tetrabenazine apparently interacts with the dopamine receptor, because this drug displaced the dopamine antagonist 3H-spiperone from dopamine receptors of the three different tissues with an apparent inhibitory constant of about 5 μM. We conclude that tetrabenazine has biological and pharmacological properties typical of a dopamine receptor antagonist. These observations should stimulate a reevaluation of the mechanisms for the actions of tetrabenzine previously attributed exclusively to a “reserpine-like” effect.