Elevated expression of c-fos mRNA and/or its protein (c-Fos) has been repeatedly observed following activation of brain cells with various stimuli. As a consequence it has been proposed that elevated c-fos expression may serve as a marker of neuronal activation reflecting a predisposal to long-term changes in cell functioning (neuronal plasticity). In studies on copulatory behaviour of male animals, elevated Fos immunoreactivity (related most probably to c-Fos) has been used by many researchers as a tool to reveal functional neuroanatomy of the behaviour. They found that brain regions such as medial preoptic area (MPOA), bed nucleus of stria terminalis (BNST), medial amygdala (MeAm) and central tegmental field (CTF) are always activated in a copulation-dependent manner in various species (rats and hamsters were the most often studied in this regard). This paper reviews the data gathered in this field, and discusses them in the context of a role of elevated Fos expression in acquisition vs. performance of the sexual behaviour.