There appear to exist two sites on pituitary thyrotrophs which recognize TRF; one with an affinity constant of 2 X 10-8M, the other with a constant of ca. 5 X 10-7M. The high affinity site saturates at a concentration of TRF tenfold greater than that required to obtain maximal biological activity, possibly indicating the presence of a large number of spare TRF receptors. The TRF binding sites have affinities for TRF analogues which are proportional to their biological potencies as determined by bioassays. An analysis of the competition binding data shows that the two binding sites have slightly different structural preference for some of the TRF analogues. (Endocrinology92: 1629, 1973)