1. The haemodynamic changes during the development of hypertension in rabbits have been studied for 1 week before and 4–5 weeks after sham-operation (thirteen rabbits) or bilateral renal cellophan wrapping (thirteen rabbits), Doppler flowmeters being used to measure cardiac output.
2. There was a significant but transient rise in cardiac output on post-operative day 4 in six of thirteen sham-operated rabbits and six of thirteen renal-wrap rabbits before development of hypertension. Both groups showed the same rise in mean cardiac output of 10%. The rise in cardiac output was more transient than in other species, and was not essential for subsequent development of hypertension.