The effects of ethaverine on serotonin biosynthesis in murine mastocytoma P815 cells were investigated. Ethaverine at 2.5${sim}$10 ${mu}$M decreased serotonin content in a concentration-dependent manner. The IC$_{50}$ value of ethaverine was 6.1 ${mu}$M. Ethaverine at concentrations up to 20 ${mu}$M was not cytotoxic towards P815 cells. Under these conditions, tryptophan hydroxylase (EC 1.14.16.4; TPH), a rate-limiting enzyme in serotonin biosynthesis, was inhibited by ethaverine in P815 cells (15.3% inhibition at 7.5 ${mu}$M), however, aromatic L-amino acid decarboxylase (EC 4.1.1.28) was not. These results indicate that ethaverine leads to a decrease in serotonin content by reducing TPH activity in P815 cells.