Bromocriptine, a dopaminergic agonist, inhibited the growth of human small cell lung cancer (SCLC) implanted as tumor xenografts in athymic nude mice; the effect was dose dependent. In mice bearing a SCLC with ectopic vasopressin production, plasma levels of human vasopressin-associated neurophysin decreased concomitantly. Electron microscopy of tumor tissues revealed marked degenerative changes, including pyknosis, densely aggregated chromatin masses, and vacuolization of cytoplasm after bromocriptine treatment. When a SCLC cell line, NCI-H69, was grown in semisolid medium, bromocriptine inhibited its clonal growth in a dose-related manner. Coincubation with dopamine D2 receptor antagonist, metoclopramide, or domperidone, completely blocked the inhibitory effect of bromocriptine. Receptor studies with a dopamine D2 receptor ligand, [125I]idosulpride, showed high affinity binding sites on the membranes of SCLC cells. These results indicate that SCLC cells are enriched with dopamine D2 receptors, which may mediate the growth-inhibitory effect of bromocriptine on SCLC. Dopaminergic agonists may be useful in the medical treatment of SCLC.