Male mice with targeted deletion of the genes encoding the neuronal (NOS -1K/K or nNOS K/K ) isoform of nitric oxide synthase display altered aggressive behaviors. Male nNOS -1K/K mice are more aggressive than wild-type (WT) mice in all testing paradigms. Testosterone is necessary, but not sufficient, for evoking the persistent aggression, and that serotonin (5-HT) metabolism is altered in male nNOS -1K/K mice. The specific deletion of the nNOS -1 gene not only results in a lack of nNOS -1 protein, but in common with many genes, affects several 'downstream' processes. In this review, we address whether the elevated aggression in male nNOS -1K/K mice reflects pleiotropic effects of the nNOS -1 gene on pain sensitivity, 'anxiety-like', or 'depressive-like' behaviors. For example, male nNOS -1K/K mice display increased sensitivity to painful stimuli, which may prolong aggressive interactions. Despite elevated corticosterone concentrations, nNOS -1 knockout mice appear to be less 'anxious' or fearful than WT mice. Male nNOS -1K/K mice display longer latencies to right themselves on an inverted platform and spend more time in the center of an open field than WT mice. Because of reduced serotonin turnover, the excessive aggressiveness displayed by nNOS -1K/K mice may be symptomatic of a depressive-like syndrome. However, nNOS -1K/K mice rarely display behavioral 'despair' when assessed with the Porsolt forced swim test; rather, nNOS -1K/K mice show vigorous swimming throughout the assessment suggesting that the aggressive behavior does not represent depressive-like behavior. Importantly, aggressive behavior is not a unitary process, but is the result of complex interactions among several physiological, motivational, and behavioral systems, with contributions from the social as well as the physical environment. Lastly, the multiple, and often unanticipated, effects of targeted gene disruption on aggressive behavior are considered.