The experiments described in this review reveal that the expression and modulation of aggressive responses in the cat are organized by two distinct sets of pathways. One set of pathways is associated with the elicitation of a specific form of attack behavior. It includes the medial hypothalamus and its projections to the PAG for the expression of defensive rage behavior and the lateral hypothalamus and its descending projections for the expression of predatory attack behavior. The primary focus of the present review is upon the analysis of defensive rage behavior. It was demonstrated that the pathway from the medial hypothalamus to the PAG, which appears to be essential for elicitation of defensive rage, is powerfully excitatory and utilizes excitatory amino acids that act upon NMDA receptors within the PAG. The other pathways examined in this review arise from different nuclei of the amygdala and are modulatory in nature. Here, two facilitatory systems have been identified. The first involves a projection system from the basal complex of amygdala that projects directly to the PAG. Its excitatory effects are manifest through excitatory amino acids that act upon NMDA receptors within the PAG. The second facilitatory pathway arises from the medial nucleus of the amygdala. However, its projection system is directed to the medial hypothalamus rather than the PAG. Its neurotransmitter appears to be substance P that acts upon NK1 receptors within the medial hypothalamus (see Figure 10). It has yet to be determined whether substance P acts upon any of the other neurokinin receptor subtypes. It should also be pointed out that the substance P pathway from the medial amygdala to the medial hypothalamus functions to suppress predatory attack behavior elicited from the lateral hypothalamus. In this network, it is likely that the modulatory effects of the medial amygdala require the presence of a second, inhibitory pathway from the medial hypothalamus that innervates the lateral hypothalamus. At the present time, the neurochemical nature of this second pathway remains unknown, although it is suggested that such neurons may be GABAergic. One major inhibitory pathway was also identified. It arises principally from the central nucleus of the amygdala and projects to the PAG. Its powerful suppressive effects upon PAG elicited defensive rage behavior are mediated through opioid peptides that act upon mu receptors within the PAG. While the present series of studies have begun to define the structural and functional nature of the neural systems that regulate aggressive behavior, our understanding of the overall mechanisms regulating different forms of aggressive behavior remains incomplete.(ABSTRACT TRUNCATED AT 400 WORDS)