It is accepted that social stress in relation to confrontation and competition can elicit behavioural and hormonal changes in social mammals. These effects have, however, been less frequently studied among female-female interactions. In the present study female-female confrontation experiments were carried out to monitor socio-positive and agonistic behaviour by controlling for the oestrus cycles of 12 individuals. Additionally, plasma cortisol (CORT) and testosterone (T) levels were determined before and after the experiments. During non-oestrus conditions a significant increase in CORT levels from pre- to post confrontation was registered and females spent more time to sit side by side. During vaginal oestrus the confrontation experiments revealed avoiding of a conspecific female by showing increased flight behaviour. However, during that period no changes in CORT levels were found. But, a non-significant increase in T was measured from pre- to post confrontation in both cycle phases, while no differences in the display of aggressive behaviours were found. These findings indicate considerable influences of different oestrus cycle phases on social stress-induced CORT secretion and the modulation of socio-positive and agonistic behaviour in female guinea pigs.