maintenance of blood pressure. We report the effect of noradrenaline infusion on blood pressure and plasma noradrenaline concentration following oral administration ofclonidine (300 tg) to four normal subjects.Intravenous cannulae were inserted into each anticubital fossae; a 3 in catheter on the left and a 12 in catheter on the right. Blood pressure was measured using a Roche Arteriosonde 1217 with the pressure cuff on the right arm. Blood for plasma noradrenaline measurements was drawn from the left arm and noradrenaline was infused through the right arm cannula. Noradrenaline infusion was started 4 h after clonidine administration and the rate of infusion was increased in a stepwise fashion. Rates of 0.01, 0.03, 0.05, 0.075, 0.10 and 0.200 tg kg-' min-' were used and infusion at each rate was continued for 10 min. Noradrenaline administration was stopped when the systolic blood pressure reached a value 30% greater than the pre-dose value. Pulse rate was measured continuously, blood pressure was measured every 2 min during the noradrenaline infusion and blood for noradrenaline was withdrawn before clonidine, 4 h after clonidine and at the eighth and tenth min of each infusion.The 1.44+0.54ng/ml in the four subjects. There were marked differences between subjects in the plasma noradrenaline levels achieved at any given infusion rate; at 0.10 tg kg-' min-' the mean value was 2.62 + 0.72 with a range of from 0.85 ng/ml to 4.35ng/ml. The sensitivity of blood pressure to changes in plasma noradrenaline also varied markedly between subjects. These results show that restoration of the concentration of noradrenaline in plasma to control levels does not reverse the hypotensive effect of clonidine. Following oral or intravenous administration of this agent, circulating noradrenaline does not appear to be responsible for the delayed fall in blood pressure. Previous studies have shown that inter-individual differences in response to the hypotensive adrenergic blocking drug debrisoquine are due to individual variations in the rates of metabolism of the drug (Angelo, Dring, Lancaster, Latham & Smith, 1975;Angelo, Dring, Lancaster & Smith, 1976 For absorption studies, ten subjects received bethanidine (30 mg orally) and the amount of unchanged drug excreted in the 24 h urine was analysed by g.l.c. using an adaption of the method described by Hengstmann, Falkner, Watson & Oates (1974). The amount of unchanged bethanidine recovered in the