The degradation of the TRH by plasma was studied using a sensitive method for the separation of TRH and products formed by high-pressure liquid chromatographic analysis (HPLC). The detected products are TRH-3H or synthetic TRH. The HPLC analysis is performed on a microparticulate (10 mu) silica gel chromatographic column with CH3CN/0,01 M NH4OAc, at pH 6 (30/70) (V/V). This technique provides a good separation of TRH and the other products of degradation. In our experimental conditions, the human plasma degrades 62.5 +/- 5% of of TRH in 60 mn; very similar results are obtained with thin layer chromatography.