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1. Single units were recorded extracellularly from the preoptic and anterior hypothalamic areas of male and female rats anaesthetized with urethane. 2. The effect of single shock stimulation of both the mediobasal hypothalamus and corticomedial amygdala upon unit activity was assessed for all cells. Neurones were designated either antidromically invaded, orthodromically excited or inhibited, or non-responsive. 3. The distribution of responses among recorded cells was analysed for two groups of neurones. In one group the intercell distance was less than 100 mum and in the second group the distance was more than 1 mm. 4. For widely separated cells the connexions with the corticomedial amygdala and mediobasal hypothalamus were distributed at random. 5. In the groups of cells clustered closely together a marked structuring of connexions was apparant. In particular, there was a significant correlation between orthodromic responses from the amygdala (P less than 0-05) and cells antidromically activated and orthodromically excited by stimulation of the mediobasal hypothalamus were frequently adjacent to one another (P less than 0-02. 6. A complimentary analysis demonstrated that cells in the ventral half of the recording zone received significantly more inhibitory inputs from both the mediobasal hypothalamus (P less than 0-001) and the corticomedial amygdala (P less than 0-02) than those situated more dorsally. 7. The data provide evidence for structuring and orderliness in hypothalamic connexions that is often not apparent from descriptions of electrophysiological experiments.