To investigate the most early events occurring during the adaptation of the pituitary-thyroid axis to iodine depletion, two rat populations were submitted to 4 week low iodine regimens of different severity. The variations of the following parameters were studied: pituitary thyrotropin (TSH) concentration, serum thyroxine (T4), triiodothyronine (T3) and TSH, thyroid weight and thyroid iodine concentration. In the first population, mildly iodine depleted, serum and pituitary TSH remained unchanged. The weight of the thyroid increased by the 12th day. Serum T4 dropped by the 26th day. Serum T3 tended to increase during the whole observation period. In the second population, more severely iodine depleted, the increase in thyroid weight appeared by the 4th day. Serum T3 increased from day 13 to day 20, then returned to normal. Serum TSH increased and serum T4 decreased by the 20th day. These results suggest that, in the adaptation to iodine deficiency in the rat, autonomous thyroid regulatory mechanisms play a major role at the onset of goiter growth. On the other hand, most likely a combined effect of serum T3 and T4 triggers variations in pituitary TSH secretion.