1. The role of the autonomic nervous system in the control of thyroxine release from the rat thyroid gland in vitro, has been investigated using electrical field stimulation as a device to induce intrathyroidal nerves to release their neurotransmitters.
2. The effects of field stimulation on follicular cell membrane potential and input resistance have been investigated using continuous intracellular recordings from individual cells.
3. Electrical field stimulation promotes up to an eightfold increase in throxine release from the thyroid gland in vitro. This effect is blocked by the neurotoxin, tetrodotoxin.
4. Electrical field stimulation has no effect on follicular cell membrane potential or input resistance. This is consistent with the results of previous pharmacological experiments using this preparation.
5. It is concluded that electrical field stimulation induces intrathyroidal nerves to release their neurotransmitters, and that the net effect of this is to cause thyroxine release from the thyroid follicles. The existence of a directly acting secretomotor innervation to the thyroid follicle is suggested.