The uptake of [3H]octopamine, [3H]norephedrine and [3H]phenylethanolarnine in slices of cerebral cortex and heart of the mouse was investigated. Cocaine inhibited the uptake of octopaniine but had no effect on that of the other two arnines, whose uptake seems to be a pure physico-chemical partition between the tissue and the solution. Together with previous results, these indicate that the cocaine-sensitive uptake is linked with the hydroxyl groups in the benzene nucleus and particularly with that in the rneta position. The results bear out the view that the potentiating effect of cocaine on the direct effect of sympathomimetic amines is due to inhibition of the uptake of the amines in sympathetic nerves, but they contradict the assumption that the antagonizing effect of cocaine on the indirect action of the amines is due to inhibition of the amine uptake.N previous experiments it was found that the uptake of tritiated I sympathomimetic amines in brain slices which had been incubated with the amines for a short time was greatly reduced by cocaine and desipramine but not by reserpine (Ross & Renyi, 1966a, b, c). This indicates that under these conditions the uptake of the amines via the neuron membranes of adrenergic nerves was the most important factor governing the amine accumulation. The rate of uptake was found to be dependent on the chemical structure of the amines. For instance noradrenaline and dopamine were taken up with similar velocities while tyramine showed a slower uptake. (-)-Amphetamine was not actively taken up at all.To obtain more information the investigation has been extended with three amines, namely [3H]octopamine, [3H]norephedrine and [3H]phenylethanolamine.