Thyroid hormone action in the brain is strictly regulated, since these hormones play a crucial role in the development and physiological functioning of the central nervous system. Hormone kinetics and molecular events at the nuclear receptor level during the adaptation of the brain of chicken to hypothyroidism were simultaneously investigated. Data obtained by Oldendorff's 'single-pass' technique showed a significantly higher labelled 3,3',5-triiodothyronine (I-125-T-3) uptake into the brain of surgically thyroidectomized (TX) 2-week-old broilers after 1 week of the surgery in comparison to sham-operated (SH) and T-3 supplemented (TX + T-3) controls in the 10th second after the bolus injection. Telencephalons showed the highest, while cerebellum the lowest uptake intensity in all groups. In a similar arrangement of experiments the expression of the TR alpha- and TR beta nuclear thyroid receptors in the telencephalon of TX and control chickens was investigated by a semiquantitative RT-PCR study in the following way: cDNAs were standardised using a quantitative RT-PCR-based approach for beta-actin, then amplified for thyroid receptors. The level of both the TR alpha and TR beta coding mRNA was elevated in hypothyroidism. In conclusion, the presented hormone kinetics and TR expression data provide further details of the cellular and molecular events occurring during the adaptation to hypothyroidism of the brain of chicken.