The concentration of TRF in cerebrospinal fluid (CSF) of the 3rd ventricle and content of TRF in median eminence of the rat was examined. CSF (0.3–0.5 μl per animal) was collected from the 3rd ventricle by a microcannula technique and pooled samples from 10–15 animals were examined for TRF using an in vitro pituitary assay method. TRF concentration in 3rd ventricle CSF of normal male rats was 18.5 ± 4.2 pg/μl; median eminence contained 115 ± 12 pg. Cold exposure (4°C) for 16–18 h and thyroxine treatment (2.5 μg/day) for 5 days markedly reduced TRF concentration in CSF and content in the median eminence. Treatment with the anti-thyroid drug methimazole (0.01 % in the drinking water) for 5 days did not notably affect TRF in CSF or median eminence.