Arginine vasotocin (AVT), a neuropeptide, appears to influence mating and aggressive behaviours of males classically thought to be controlled by testosterone. We examined the effects of AVT on the calling behaviour and competitive ability of the grey treefrog Hyla versicolor under natural field conditions. AVT increased the probability that males produced advertisement calls from a resident's calling site. Most intriguing, AVT increased an intruder male's ability to acquire calling sites from resident males without physical aggression in staged encounters. This is particularly significant because resident males typically win encounters over intruders. This is the first study demonstrating that AVT manipulations in a natural population can directly influence the outcome of male-male competition for a resource and reverse the resident-male advantage. Copyright 1998 The Association for the Study of Animal Behaviour.