Galanin (GAL) is a 29-amino acid peptide implicated in neuroendocrine regulation of prolactin, growth hormone and thyrotropin in the rat. GAL-like immunoreactivity and GAL messenger RNA (mRNA) are present in the anterior pituitary (AP) and hypothalamus and the expression of GAL mRNA has been shown to be modulated by peripheral gonadal steroid hormones. In view of possible interactions between members of the steroid/thyroid hormone receptor family and recent data suggesting an effect of GAL on thyrotropin secretion, we investigated the possible influence of thyroid status on GAL concentrations in the hypothalamus and AP of the male rat. Three weeks after the surgical removal of the thyroid gland from male rats, the concentrations of GAL in the median eminence (ME) and AP were reduced 54 and 65%, respectively. Similarly, GAL concentrations were decreased 39% in the ME and 69% in the AP of animals rendered hypothyroid by treatment with propylthiouracil (PTU). The effects of PTU treatment in both regions were reversed by daily T4 injections (50 µg/kg). The effects of PTU in the ME were reversed after 2 weeks of T4 treatment, whereas 3 weeks of replacement therapy were required to restore GAL concentrations in the AP. However, T4 treatment of intact control animals did not influence GAL concentrations. This study demonstrates that the presence of thyroid hormones is required for the maintenance of physiological concentrations of GAL in the hypothalamus and AP of the rat. These data also suggest that GAL may be involved in the negative feedback regulation of the hypothalamohypophysial-thyroid axis.