Abstract. A technique for in vivo infusion in the superior thyroid artery in rats and mice was developed and evaluated. The influx catheter is inserted in retrograde direction into the superior carotid artery. The infusate mixed with blood is directed exclusively to the thyroid lobe via the superior thyroid artery. The thyroid isthmus is divided and the other lobe serves as a control. Thyroid ultrastructure was unaltered after infusion for at least 4 h and the follicle cells displayed a normal morphological response to TSH. Electron microscopical autoradiography (125I, [3H]leucine) was performed using 20–80 times less label as compared with iv administration. Infusion of forskolin, a stimulator of adenylate cyclase, increased the intrathyroidal cyclic AMP levels about 10-fold. Infusion of the ionophore monensin yielded typical dilations of Golgi cisternae as well as reduced secretion of newly synthesized protein into the follicle lumen. The arterial infusion technique developed is useful when in vitro methods or systemic administration of sustances are unsuitable. The technique permits selective administration of small amounts of experimental substances to the thyroid in high concentrations.