Numerous papers have been published describing the effects of learning and environmental changes on the wiring of brain areas in mammals and birds. The density of dendritic spines, which can be taken as a measure of the complexity of a given neuronal network, has been shown to increase or to decrease depending on the experiment and on the brain area involved. Almost no information is available concerning the speed with which a given network reacts to learning events or environmental changes. We therefore examined the time course of spine density changes in two areas of the zebra finch forebrain, which have been shown previously to be either involved in sexual imprinting (LNH, lateral part of the neo-hyperstriatum) or to react to environmental changes (ANC, archi-neostriatum caudale). The decrease of spine density in LNH of zebra finch males after sexual imprinting is very fast, the new level of spine density is reached after 2 days. In contrast, decrease of spine density within ANC as a consequence of transferring birds from a social condition into isolation is very slow, lasting about 3 weeks. The increase of spine density within ANC after transfer of the males from isolation to a social condition occurs within 3 days. The differences in adaptation times cannot be due to limitations in the growth speed of single spines, because this has been shown to be much faster (hours instead of days). Instead, the speed of adaptation may be dependent on the availability of information about the final wiring diagram and on functional aspects like the energy demands for maintenance or alteration of a given neuronal network, or the necessity of quick adaptation to enhance the fitness of the animal.