Carcinoid tumors are neuroendocrine malignancies that frequently metastasize and secrete hormones that cause debilitating symptoms in patients. In this study we report the effects of valproic acid (VPA), a drug long used for the treatment of epilepsy, on the growth and neuroendocrine phenotype of human carcinoid cancer cells. VPA treatment of gastrointestinal and pulmonary carcinoid cells resulted in a dose-dependent inhibition of cancer cell growth. Western blot analysis revealed degradation of cyclin D1 and an increase in cyclin-dependent kinases p21 and p27 with VPA treatment. Flow cytometry confirmed that the mechanism of VPA-induced growth inhibition is G(1) phase cell cycle arrest. Furthermore, VPA suppressed expression of the neuroendocrine tumor marker chromogranin A. In addition to these effects, VPA also increased levels of full-length Notch-1 and the active Notch-1 intracellular domain. Luciferase reporter assays incorporating the centromere-binding factor 1 (CBF-1) binding site and the achaete-scute complex-like 1 (ASCL-1) promoter confirmed the functional activity of VPA-induced Notch-1. Transfection of Notch-1 small-interfering RNA into carcinoid tumor cells blocked the effects of VPA on Notch-1 activation, ASCL-1 suppression, p21 induction, and cell growth inhibition. VPA also suppressed growth of carcinoid tumors in vivo in a mouse tumor xenograft experiment. These findings confirm the important role of Notch-1 in regulating the growth and neuroendocrine phenotype of carcinoid tumor cells. On the basis of this study, a clinical trial of VPA for patients with advanced carcinoid cancer will be conducted.