Responses to hemithyroidectomy and thyroxine treatment were examined in normal kittens 5–21 days of age and in kittens in which the basal hypothalamus was deafferented. After hemithyroidectomy on the 5th day, the remaining thyroid lobe exhibits no compensatory increases in weight or <sup>131</sup>I uptake. Deafferentation at 5–6 days of age evokes a marked increase in gland weight during the next 15 days. Treatment with thyroxine (0.25–5.0 µg/kg/day) for 15 days causes an almost 3-fold increase in gland weight, and high PBI levels. Thyroxine: cholesterol pellets that had no effect subcutaneously markedly stimulated gland weight when implanted in the preoptic area of the hypothalamus. It is concluded that, as in adult cats, the preoptic area of the hypothalamus in kittens contains an inhibiting neural pathway associated with regulation of TSH secretion, but that the relevant neurone pools in this area of newborn and adult cat brains respond to thyroxine in opposite manners.