1. Clonidine (6 mg of base/1 of water) was given as drinking fluid to normotensive rats or rats with established or early hypertension.
2. Spontaneous hypertensive rats (6 months old: average dose of clonidine, 0·6 mg 24 h−1 kg−1) showed a sustained fall in blood pressure over 3 weeks.
3. The same clonidine solution given for 6 weeks to two-kidney Goldblatt rats with early-stage hypertension (average dose of clonidine: 1 mg 24 h−1 kg−1) or spontaneously hypertensive rats (clonidine dose: 1 mg) induced a fall in mean blood pressure, but no change in normotensive rats.
4. Replacement of clonidine by water induced hypertension and lability which led to death in hypertensive but not in normotensive rats.