1. Systemic and regional haemodynamics were determined in conscious Wistar-Kyoto and spontaneously hypertensive rats with the radioactive microsphere technique after sham lesion or bilateral electrolytic lesion of the nuclei of the anterior hypothalamus.
2. Anterior hypothalamic lesions produced an increase in blood pressure, heart rate and cardiac output.
3. In the rats with lesions there were changes in regional haemodynamics: heart and skeletal muscle flow increased; renal, skin and splanchnic flow decreased.
4. The haemodynamic consequences of ablation of the anterior hypothalamus were similar in both normotensive Wistar-Kyoto and spontaneously hypertensive rats. The haemodynamic pattern resembles that seen during fighting and exercise.