Blood pressure overshoot under pentobarbitone anaesthesia following a single dose of clonidine in rats

1. Withdrawal of chronic antihypertensive therapy with clonidine is known to produce a blood pressure overshoot. It has been reported that the same may occur after a single dose of clonidine. 2. A single, intramuscular dose of clonidine (0.05 mg/kg) produced an overshoot in blood pressure, on the day following administration, in normotensive rats anaesthetized with sodium pentobarbitone. 3. No rebound elevation of mean arterial pressure or of heart rate occurred in conscious, normotensive, spontaneously hypertensive or renal hypertensive rats following this dose of clonidine, nor did it occur in rats anaesthetized with ether. 4. It is suggested that the overshoot phenomenon in rats under barbiturate anaesthesia may involve an interaction between an effect of clonidine and the barbiturate.