The identity and distribution of amines in the calf thyroid was investigated by histochemistry and cytospectrofluorometry, and the possible significance of these amines in the synthesis of thyroid hormone was assessed by testing their influence on the uptake and organifkation of iodine in calf thyroid cells, dispersed by continuous-flow trypsinization. The calf thyroid was found to contain four different amine-storing cell systems: 1. Norepinephrine (NE) was present in sympathetic nerve fibres that were located both as a network around vessels and as single fibres running close to the follicles. 2. Dopamine (DA) and histamine (Hi) were demonstrated in mast cells that were numerous and occurred both in perifollicular and perivascular spaces. 3. DA was also found in parafollicular cells (C cells), which were relatively few. 4. Numerous acetylcholine-(ACh)-containing nerve fibres were seen around vessels and between follicles. Histochemical examinations of smears of thyroid cell suspensions revealed that, apart from follicle cells and erythrocytes, well-preserved mast cells and C cells were present in these suspensions. Incubations of isolated thyroid cells with various amines at 5 × 10-5M for 1 hr showed that, of the four amines normally present in the calf thyroid, NE and DA, but not Hi or ACh, consistently stimulated the uptake and organification of iodine. At the same dose level, also epinephrine, isoproterenol, and S-hydroxytryptamine yielded responses similar to those induced by NE and DA. The organification of iodine in response to each of these five aromatic monoamines was inhibited by the alpha adrenergic blocking agent phentolamine, whereas the uptake of iodine was less markedly influenced. None of the responses was reduced by the beta adrenergic blocking agent L-propranolol. Incubations of isolated thyroid cells with compound 48/80 resulted in a depletion of DA and Hi from mast cells in the suspension, and an increase of the uptake and organification of iodine. The drug did not seem to affect the C cells. It is suggested that aromatic monoamines can stimulate the synthesis of thyroid hormone, by a direct action on alpha adrenergic receptors in the follicle cells. It seems possible that, by such an action, NE released from intrathyroidal sympathetic terminals, and DA released from intrathyroidal mast cells, may participate in the regulation of the synthesis of thyroid hormone. (Endocrinology93: 193, 1973)