The elcctrogram of the thyroid gland in normal guinea pigs shows a lasting monophasic reaction beginning 1–2 minutes after the intravenous injection of thyrotrophin (TSH). With a dose above 1.6 IU of TSH no relation between reaction and dose has been observed. Distinct changes of the electrocardiogram may be observed after intravenous injection of more than 12.5 IU of TSH. Probably they are due to the presence of non-specific proteins. The early reaction in the electrogram of the thyroid gland after injection of TSH is interpreted as a consequence of the stimulated secretion of thyroid hormones.