Histidylproline a metabolite of Thyrotrophin Releasing Hormone specifically binds to both high and low affinity sites in fresh rat brain membranes. Characterisation of the high affinity site under optimal conditions demonstrated an equilibrium dissociation constant (KD) of approximately 9nM and maximum binding capacity of approximately 120 fmols/mg protein. Kinetic analysis of [3H]-His-Pro binding is limited by low binding density, instability of the high affinity site and rapid degradation of the radioligand. The thiol blocking reagent pHydroxymercuriphenylsulphonic acid (HMPS) inhibited [3H]-His-Pro degradation but also reduced binding of the peptide to membranes. The results are discussed with reference to the lack of specific binding sites in brain for the proposed neuropeptide and TRH metabolite cyclo(His-Pro).