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Notes: Summary 1. Isolated rabbit hearts were perfused with (−)-noradrenaline, (−)-adrenaline and (±)-isoprenaline for various time periods (1–180 min) and then washed with an amine-free medium. The venous concentration of the amine was estimated fluorimetrically during the infusion and after its end, to study removal and efflux, respectively. 2. In untreated hearts and after pretreatment with reserpine the removal had a constant rate over 20–60 min. After pretreatment with pargyline to block monoamine oxidase (MAO), however, the removal of noradrenaline declined exponentially to zero. Inhibition of the neuronal uptake (desipramine) and chemical sympathectomy (6-hydroxydopamine) abolished the removal of noradrenaline. Isoprenaline was not removed to any significant extent. 3. The efflux of noradrenaline originated in 4 different compartments as indicated by the various efflux components. The half-times in untreated hearts were about 0.1, 0.4 and 7 min, and after block of intraneuronal inactivation (pargyline plus reserpine) 0.1, 0.4 and 43 min. 4. The 1st compartment (t 1/2=0.1 min) represents an open compartment including the vascular space. Several results indicate that the 2nd compartment is identical with the extracellular space. Inhibition of amine uptake by desipramine caused a strong increase of the total output resulting from the 2nd phase of efflux which then contributed a fraction of 0.21 to the total initial rate of efflux. Furthermore, endogenous noradrenaline released by sympathetic nerve stimulation and by DMPP was washed out of the heart into the perfusate with nearly the same half-time (t 1/2=0.32 min) as that found for the 2nd phase of efflux following noradrenaline infusion (t 1/2=0.41 min). The 3rd compartment of untreated hearts (t 1/2=7 min) is not identical with that found after block of both MAO and vesicular uptake (t 1/2=43 min). The former compartment was smaller and more rapidly equilibrated, and access of catecholamines was not depressed by inhibition of neuronal uptake or by chemical sympathectomy. On the other hand, the compartment occurring after block of intraneuronal inactivation became smaller—or even disappeared—after inhibition of neuronal uptake or after chemical sympathectomy; this latter compartment was not affected by block of vesicular uptake. It is concluded that the 3rd compartment of untreated hearts is located extraneuronally; an intraneuronal compartment could not be detected in these hearts under efflux conditions. The 4th noradrenaline compartment, occuring as 3rd phase of efflux after block of MAO, is located within the adrenergic nerves but outside the vesicles; therefore this compartment is identical with, or is part of, the axoplasm. 5. The efflux of isoprenaline originated in 3 different compartments of which the 1st and 2nd (t 1/2=0.1 and 0.3 min) seem to be identical with the corresponding noradrenaline compartments. The 3rd isoprenaline compartment must be extraneuronal since the slow isoprenaline efflux, even after block of MAO, was similar to that of noradrenaline from untreated hearts. In contrast, after block of intraneuronal inactivation the slow adrenaline efflux was identical with the neuronal efflux of noradrenaline. 6. Since the half-time of elimination of noradrenaline from the axoplasmic compartment increased with increasing initial rates of efflux (Y o), it was assumed that a capacity-limited process was involved in the neuronal efflux. This can be exhaustion of intraneuronal binding sites, saturation of efflux or enzymatic degradation.