1. The thyroid axis in developing lungfish is being explored to ascertain whether it shows similar deficiencies to those characteristic of neoteny in urodele amphibians. 2. At hatching, the pituitary of Neoceratodus forsteri comprises a lumen surrounded by a single layer of epithelial cells lying immediately below the hypothalamus, but unconnected to it. Over the first year of development, the number of pituitary cells increases and several cell types, including thyrotropes, can be recognized immunocytochemically, but the pituitary remains unconnected to the hypothalamus. 3. By treating the lungfish with thyroid inhibitor, no increase in thyroid uptake of iodine, indicative of induced pituitary thyroid-stimulating hormone (TSH) activity, could be demonstrated; neither was there any response to exogenous human TSH. In the liver, thyroid hormone receptors were found to be primarily of the alpha type. 4. Taken together, these findings suggest that up to 1 year of age, lungfish development is equivalent to amphibian premetamorphosis, which is consistent with neoteny but cannot be taken as evidence for neoteny unless confirmed at later stages of lungfish life history, which are yet to be studied.