It has been shown that thyroid hormone stimulates the activity of alkaline phosphatase, a marker of mature osteoblast phenotype, in osteoblasts. In the present study, we investigated whether p44/p42 mitogen-activated protein. (MAP) kinase is involved in the thyroid hormone-stimulated alkaline phosphatase activity in osteoblast-like MC3T3-E1 cells. Triiodothyronine (T-3) markedly induced the phosphorylation of p44/p42 MAP kinase. PD98059 and U0126, inhibitors of the upstream kinase that activates p44/p42 MAP kinase, significantly enhanced the T-3-induced alkaline phosphatase activity in a dose-dependent manner. The phosphorylation of p44/p42 MAP kinase induced by T-3 was reduced by U0126. These results strongly suggest that p44/p42 MAP kinase takes part in the thyroid hormone-stimulated alkaline phosphatase activity in. osteoblasts and that p44/p42 MAP kinase plays an inhibitory role in the thyroid hormone-effect.