The parafollicular cell system of the mouse thyroid gland has been studied with specific histo-chemical methods for the cellular localization of certain monamines and their immediate precursors as well as of monoamine oxidase. Dopa-mine and the precursor 3, 4-dihydroxyphenylalanine (DOPA) were administered in combination with drugs interfering with certain enzymatic processes in the metabolism of monoamines. In untreated mice the parafollicular cells contain neither catecholamines nor 5-hydroxytrypt-amine. However, the parafollicular cells (but not the follicular epithelium) can selectively take up exogenous L-DOPA (and also dopamine, but not D-DOPA), which is then decarboxylated in the cells to a primary catecholamine. The amine that can be induced in the parafollicular cells is stored in high concentrations by a mechanism which is blocked by reserpine. Monoamine oxidase, also found to be present in parafollicular cells, is active in the breakdown of the amine formed (or directly taken up) in the parafollicular cells. Thus, monoaminergic mechanisms in the specific system of parafollicular cells in the mouse thyroid gland can be demonstrated, although these cells normally do not contain any monoamines demonstrable with available histochemical methods.