The aggregation of melanosomes within melanophores (pigment cells) attached to isolated scales from the cuckoo wrasse (Labrus ossifagus L.) can be elicited either by stimulation of intrinsic nerves (electrical field stimulation of K+ depolarization) or by adrenoceptor agonists. The melanophores of isolated scales survived in a culture medium for up to 20 days. During this period a progressive increase in sensitivity to exogenously applied noradrenaline took place. After 8 days of isolation there was an about 300-fold increase in sensitivity to noradrenaline. The increased sensitivity was accompanied by a progressive loss of the nerve-elicited response during the first days of isolation. Before and during this early period cocaine was found to potentiate the effect of noradrenaline about 8-fold. On the 3rd day after isolation the effect of cocaine was lost, which coincided fairly well with the time when the nerve-mediated response was lost. The early developing increase in sensitivity to noradrenaline (8-fold) is therefore most likely caused by loss of presynaptic amine uptake, but the main part (50-fold) of the increase in sensitivity presumably has other explanations, like changes in the receptor population.