The localization of neurons containing immunoreactive thyrotropin-releasing hormone (TRH) was examined in the hypothalamus of intact, propylthiouracil (PTU)-treated, and colchicine-treated adult rats. In intact animals, immunoreactive TRH neurons were occasionally found in the paraventricular and dorsomedial nuclei, and in the anterior and lateral hypothalamic areas. In PTU-treated animals, the cellular appearance of the hypothalamus with the exception of the paraventricular nucleus was almost similar to that of intact animals. In the paraventricular nucleus, only the cells localized in the periventricular and medial parvocellular subdivisions significantly increased in number and became hypertrophic in comparison with intact animals. The distribution of immunoreactive fibers in the hypothalamus was almost equal among the 3 animal groups with the exception of that in the median eminence, in which the fibers were most densely concentrated in intact animals, and most sparse in PTU-treated rats. The fibers projecting into the median eminence were distinguished into the periventricular and lateral pathways, which are derived from the neurons in the periventricular and medial parvocellular subdivisions of the paraventricular nucleus, respectively. Thus, among immunoreactive TRH neurons in the hypothalamus, only those in the periventricular and medial parvocellular subdivisions of the paraventricular nucleus may be involved in the hypothalamic-hypophysial-thyroid axis.