In the domesticated Zebra Finch, unpaired males behaved sociably towards one another, but could be provoked to fight by the sight of a female nearby; the amount of fighting shown depended on the distance to the female. Qualitatively similar aggressive behaviour was provoked between males by the sight of a male, and between a male and female by the sight of another female. However, males were less effective than females in provoking fighting between females, and pair-bonded females were more effective than non-bonded females in provoking fighting in mixed or male dyads. Homosexual pair bonds had a similar effect to heterosexual ones in stimulating attacks on a male, but had not effect on aggressive behaviour towards a female. The aggressive behaviour seen could not be due to redirection of aggression, since the stimulus properties which were necessary to provoke attack on another individual were those relevant to sexual behaviour, and were different from those necessary to elicit attack on the individual itself. It is shown that in other cases of apparent redirection of aggression in sexual contexts, a similar conclusion can be drawn, indicating a link between the motivational systems controlling sexual and aggressive behaviour. Since stimuli from an individual may both sensitize a bird to respond aggressively to stimuli from other individuals, and elicit attack by the bird on the individual itself, a distinction between these modes of action (similar to TINBERGEN'S distinction between releasing and motivational effects) is necessary.