The effects of complete and anterior disconnection of the medial basal hypothalamus (MBH( on TSH secretion in the rhesus monkey were assessed by measuring serum thyroxine concentrations. Following complete MBH disconnection, serum thyroxine concentrations declined to 50% of preoperative control levels within 7 days. In 6 of 10 animals, thyroxine levels remained depressed throughout the postoperative observation period of 60 to 150 days. In the remainder, serum thyroxine concentrations returned to control levels after 3 weeks. Anterior disconnection of the MBH had variable effects on thyroid function. In 4 of 7 animals, serum thyroxine concentrations declined for 5 days following the operation but returned to preoperative levels within the next 3 weeks. In 2 others, serum thyroxine levels did not change while in the remaining animal thyroxine concentrations increased transiently to twice preoperative levels before returning to normal values. Thus, in the monkey, as in the rat, anterior MBH disconnection has little consistent effect on thyroid function, while complete isolation of the MBH causes a moderate decline which is most readily attributable to a decrease in thyrotropin secretion. The variations in the effects of anterior and complete MBH disconnection on thyroid function could not be confidently correlated with variations in the placement of the cuts.