In 2 of 28 cats a right phrenico-sympa-thetic anastomosis resulted in an elevated B. M. R. which decreased when followed by removal of the ipsilateral thyroid lobe in one cat and by subsequent resection of the anastomosis in the other cat. The stimulation might have been produced through the pituitary. Cervical sympathec-tomy and splanchnic nerve section (to adrenals) were without effect on the action of the ant. hypophyseal thyro-trophic hormone. Doses of adrenaline, mecholyl, and pilo-carpine, in themselves ineffective in raising the B. M. R., augmented the action of the thyrotrophic hormone.