Besides their genomic effects, oestrogens, 17beta-oestradiol in particular, also activate cellular effects that may be too rapid (seconds to minutes) to result from de novo protein synthesis. Although the existence of such nongenomic actions has been extensively demonstrated in vitro, the understanding of their behavioural significance is only emerging. Recent findings provide evidence that acute oestrogen treatments significantly affect a variety of behavioural processes, including sexual behaviour, social communication and cognition. One question arising from these results concerns the source of the oestrogens mediating nongenomic effects in vivo. In this review, data collected in vitro and in vivo are presented supporting the notion that fast modulations of local testosterone aromatisation can rapidly control the local oestrogen concentration in a time frame compatible with their rapid actions. Taken together, these data provide compelling evidence of how rapid changes in the local production and action of oestrogens can shape complex behaviours.