The influence of thyroid hormones on neurons staining for a pro-TRH-derived cryptic peptide sequence in the rat hypothalamic paraventricular nucleus was assessed by using quantitative immunocytochemistry. The antiserum used for immuno-cytochemical studies recognized a cryptic fragment of pro-TRH located in the Phe 178-Ser 186 region of the precursor. Hypothyroid female rats obtained following 1 month administration of 6-n-propyl-2-thiouracil (PTU) as well as euthyroid animals were treated with a low (3 µg/kg/day) or high (67 µg/kg/day) dose of L-thyroxine (T<sub>4</sub>) for 4–10 days before sacrifice. All the animals received an intraventricular injection of colchicine 2 days prior to perfusion with 4% paraformaldehyde. PTU treatment induced an increase (45% over control) in the number of immunostained neurons. The administration of a low dose of T<sub>4</sub> to hypothyroid animals slightly increased the number of staining for a pro-TRH-derived cryptic sequence neurons, while being uneffective in euthyroid rats. In hypothyroid animals, a 4-day treatment with the high dose of T<Sub>4</sub> did not produce any changes whereas the 7-day treatment reduced the number of positive cells to 42% of control. In euthyroid rats treated with the high dose of T<sub>4</sub> for 4 and 7 days, the number of stained neurons was reduced to 60 and 37% of control, respectively. These results indicate that thyroid hormones can modify the amounts of staining for a pro-TRH-derived cryptic sequence in hypothalamic paraventricular neurons. They also suggest that thyroid hormones can regulate TSH secretion, not only at the pituitary level, but also by modifying the activity of neurons staining for a pro-TRH-derived cryptic sequence in the hypothalamic paraventricular nucleus.