The ability to assay small amounts of the peptide releasing hormones in biological fluids would aid greatly in the assessment of hypothalamic function. We have recently described a specific radioimmunoassay for luteinizing hormone releasing hormone (LH-RH) (Jeffcoate, Fraser, Gunn & Holland, 1973a, b) and in this study we report a radioimmunoassay for the tripeptide, thyrotrophin releasing hormone (TRH). Thyrotrophin releasing hormone (2 mg) was conjugated to 10 mg bovine serum albumin in borate buffer pH 9·0 by the bis-diazotized benzidene method (Bassiri & Utiger, 1972 a). After 2 h at 5 °C the mixture was dialysed against distilled water for 48 h and against 0·15 m-NaCl for 24 h. This technique conjugates TRH by the imidazole ring of histidine to the protein. A sample of conjugate (2·5 mg) in saline was homogenized with Freund's complete adjuvant and injected into 20 intradermal sites in a White New Zealand rabbit.