Plasma TSH responses in the rat, as measured by radioimmunoassay, were compared following hypothalamic electrical stimulation or intravenous administration of synthetic TRH. Positive responses occurred within 5–10 min following electrical stimulation of widely distributed regions of the hypothalamus, including the preoptic area, paraventricular nucleus, anterior hypothalamic area and the medial-basal hypothalamus. Stimulation of the cerebral cortex, fornix, lateral hypothalamus, thalamus and mesencephalon had no effect on TSH levels. The effect of TRH on pituitary and plasma TSH concentrations was examined over the range 0.1–5000 ng. A dose of 1.0 ng gave significant elevation of plasma TSH but only the largest dose (5000 ng) significantly depleted pituitary TSH levels. Both the response to electrical stimulation and to synthetic TRH administration could be blocked by prior treatment with T4 in a dose of 50 μg/100 g bw. (Endocrinology90: 1079, 1972)