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1. 3H-Noradrenaline was infused intravenously into pentobarbitone anaesthetized rabbits to reach a steady-state plasma 3H-noradrenaline level, from which the noradrenaline plasma clearance was calculated. The plasma level of endogenous noradrenaline was determined simultaneously and the rate of noradrenaline release was then derived. 2. Pargyline, amezinium, desipramine and guanethidine all reduced the noradrenaline plasma clearance. The noradrenaline release rate was decreased by desipramine, guanethidine and clonidine. 3. Adrenaline (6 nmol/kg i.v. twice) enhanced the noradrenaline release rate by 53%. This effect was apparent after the plasma adrenaline has returned to basal levels. The adrenaline levels in sympathetically innervated tissues were elevated at this time. 4. When the rabbits were pretreated with either propranolol HCl (1 mg/kg i.p.) to block beta-adrenoceptors, or desipramine HCl (1 mg/kg i.v.) to block neuronal uptake, the facilitatory effect of adrenaline was abolished. Noradrenaline (6 nmol/kg i.v. twice) had no effect on the noradrenaline release rate. 5. These findings suggest that if the sympathetic transmitter stores contain sufficient adrenaline neuronally released adrenaline may modulate noradrenaline release in vivo by activating facilitatory presynaptic beta-adrenoceptors.

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