The high affinity receptor for TRH of a rat pituitary GH3 strain in culture (Km: 4 X 10(-9) M) was found suitable to design a radio-receptor assay for TRH. Attached GH3 cells were incubated for 30 min at 37 degrees C in presence of a tracer amount of 3H-TRH (200 pg/ml, 0.54 nM) and increasing quantities of cold synthetic TRH. After extensive washings, the radioactivity associated with the cells was counted, and the log-logit transformation was applied to the results expressed as % binding (100%: binding in absence of unlabelled TRH). The standard curve is linear between 200 pg/ml and 5 ng/ml TRH. Hypothalamic or thyroid hormones were tested for their competitive effect with 3H-TRH towards the GH3 cells receptor. None of them interfered. TRH analogs were also assayed for their competition with TRH towards the receptor. Only compounds methylated on Prolineamide or Histidine could compete with TRH, but these derivatives have never been shown to occur in vivo. Because of this high specificity, we applied this radioreceptor assay to biological samples, once extracted by acidic methanol and purified by CMC column (following a technics designed by K Bauer, Berlin).