The corticotrophin releasing (CRF) activity of stalk median eminence (SME) extracts from homozygous Brattleboro (DIhomo) rats was significantly less than that of the heterozygous (DIhet) rats which, in turn, was significantly less than normal (P less than 0.01). The bio- and immunoactivities of LH-releasing hormone (LH-RH) were not significantly different. No detectable immunoactive vasopressin was found in DIhomo SME and the vasopressin content of DIhet SME was less than normal. Chromatography of an extract of SME from 20 DIhomo rats on BioGel P2 resulted in a loss of CRF activity and the emergence of two regions of CRF activity: the peak at the void volume of the column and the later eluting peak which also had some LH-RH bioactivity but no immunoactivity. The third and major CRF peak found in normal SME, which co-elutes with vasopressin, was absent from the DIhomo chromatogram. The DIhomo LH-RH chromatogram was normal. When synthetic arginine-vasopressin was added to Brattleboro SME in amounts equivalent to those found in normal SME the CRF bioactivity was dramatically potentiated. It was concluded that Brattleboro rats have a specific defect for CRF at the hypothalamic level and this appears to be genetically linked to the synthesis of vasopressin which in itself is also essential for the stability and full expression of CRF bioactivity.