1. Adrenaline, noradrenaline, isoprenaline, tyramine, phentolamine, pronethalol, histamine and acetylcholine were each tested for their ability to prevent cocaine from causing supersensitivity to catecholamines in cat spleen strips in vitro. A high concentration of one of these drugs was added to the bath 5 min before cocaine hydrochloride (10 mug/ml). The effect on subsequent responses to catecholamines was compared with the effect of cocaine in control strips in the absence of an interfering drug.2. Phentolamine completely abolished the potentiating effect of cocaine. Large doses of adrenaline or noradrenaline reduced, but did not completely prevent, potentiation. Tyramine, isoprenaline, pronethalol, histamine and acetylcholine did not prevent potentiation.3. The ability of these drugs to interfere with potentiation does not correlate well with their ability to interfere with uptake of noradrenaline. Interference with uptake by cocaine is therefore unlikely to account fully for potentiation.