Pyroglu-N31mmethyl-histidyl-prolineamide (MeTRH) is a more potent stimulator of thyrotropin and prolactin secretion than thyrotropinreleasing hormone (TRH). In this study, the immunoreactivity of MeTRH and its susceptibility to destruction by serum were investigated. When tested in radioimmunoassays employing four different anti- TRH sera, MeTRH had 25 to 60% of the immunoreactivity of TRH. Serum destroyed the immunoreactivity of MeTRH at one-half of the rate at which TRH was destroyed. Slow degradation alone would not appear to explain the potency of MeTRH.