Fasting results in reduced thyroid hormone levels and inappropriately low or normal thyroid-stimulating hormone (TSH), partly attributed to central hypothyroidism due to suppression of pro TRH gene expression in the hypothalamic paraventricular nucleus. Recently, we demonstrated that the systemic administration of leptin to fasting animals restores plasma thyroxine (T 4) and proTRH mRNA in the paraventricular nucleus to normal, suggesting that the fall in circulating leptin levels during fasting acts as a signal to hypophysiotropic neurons in the paraventricular nucleus to reset the set point for feedback regulation of pro TRH mRNA by thyroid hormone. To determine whether the effect of fasting on the hypothalamic-pituitary-thyroid axis is mediated through the hypothalamic arcuate nucleus where leptin receptors are highly concentrated, we studied the effect of fasting and exogenous leptin administration on plasma thyroid hormone levels and proTRH mRNA concentration in the paraventricular nucleus in adult animals with arcuate nucleus lesions induced pharmacologically by the neonatal administration of monosodium L-glutamate (MSG). In normal animals, fasting reduced plasma T 4 and TSH levels and the concentration of proTRH mRNA in the hypothalamic paraventricular nucleus. In contrast, neither fasting nor leptin administration to fasting MSG-treated animals had any significant effects on plasma thyroid hormone and TSH levels and proTRH mRNA in the paraventricular nucleus. These studies suggest that during fasting, the arcuate nucleus is essential for the normal homeostatic response of the hypothalamic-pituitary-thyroid axis and may serve as a critical locus to mediate the central actions of leptin on proTRH gene expression in the paraventricular nucleus.