Pyridoxal phosphate (PLP), a bioactive form of pyridoxine, dose-dependently (10-1000 μm) inhibited cell proliferation in rat pituitary MMQ and GH3 cells and in mouse AtT-20 cells. After 4 d, MMQ cell numbers were reduced by up to 81%, GH3 cell numbers were reduced by up to 64% (P < 0.05), and AtT-20 cell numbers were reduced by up to 90%. Cell proliferation rates recovered and dose-dependently reverted to control levels after PLP withdrawal. After 4 d, PLP (400 and 1000 μm) decreased [3H]thymidine incorporation by up to 71% (P < 0.05). PLP (400-1000 μm) reduced GH3 cell GH and prolactin secretion and AtT-20 cell ACTH secretion (adjusted for cell number) by approximately 70% after 2 d. The 100 μm PLP also inhibited prolactin secretion (65%, P < 0.05) in primary rat pituitary cells treated for 2 d. PLP decreased the percentage of AtT-20 and GH3 cells in S phase and increased those in G0-G1 phase. Furthermore, PLP induced AtT-20 and GH3 cell apoptosis (28 vs. 6, P < 0.05; 26 vs. 3, P < 0.05, respectively) and dose-dependently reduced content of the antiapoptosis gene Bcl-2. These results indicate that pharmacological doses of PLP inhibit pituitary cell proliferation and hormone secretion, in part mediated through PLP-induced cell-cycle arrest and apoptosis. Pyridoxine may therefore be appropriate for testing as a relatively safe drug for adjuvant treatment of hormone-secreting pituitary adenomas.