AbstractStudies designed to determine the respective roles of substance P, excitatory amino acids, and enkephalins in amygdaloid modulation of defensive rage behavior in the cat are presented. The basic design of these studies involved three stages. In stage I, cannula electrodes for stimulation and drug infusion were implanted into medial hypothalamic or midbrain periaqueductal gray (PAG) sites from which defensive rage behavior could be elicited. Then, a stimulating electrode was implanted into a site within the medial, basal, or central nuclear complex from which modulation of the defensive rage response could be obtained. Amygdaloid modulation of defensive rage was determined in the following manner: it employed the paradigm of dual stimulation in which comparisons were made of response latencies between alternate trials of dual (i. e., amygdala = medial hypothalamus [or PAG]) and single stimulation of the hypothalamus or PAG alone. Thus, stage I established the baseline level ofmodulation (i. e., facilitation or suppression of defensive rage) in the predrug stimulation period. In stage II, a selective or nonselective receptor antagonist for a given transmitter system was administered either peripherally or intracerebrally at the defensive rage site, after which time the same dual stimulation paradigm was then repeated over the ensuing 180 min postinjection period in order to determine the effects of drug delivery upon amygdaloid modulation of defensive rage. Stage III of the study took place at the completion of the pharmacological testing phase. The retrograde axonal tracer, Fluoro‐Gold, was microinjected into the defensive rage site within the medial hypothalamus or PAG, and following a 6‐14 day survival period, animals were sacrificed and brains were processed for histological and immunocytochemical analyses for the neurotransmitters noted above. This procedure thus permitted identification of cells within the amygdala which were labeled retrogradely and which were also immunostained positively for substance P, excitatory amino acids, or enkephalin.For studies involving substance P, defensive rage was elicited from the medial hypothalamus and for studies examining the roles of excitatory amino acids and enkephalin, defensive rage was elicited from the PAG. In the first study, facilitation of hypothalamically elicited defensive rage was obtained with dual stimulation of the medial nucleus of the amygdala. In separate experiments, the selective NK1non‐peptide antagonist, CP 96,345, was administered both peripherally as well as intracerebrally into the hypothalamic defensive rage sites in doses of 0.5‐4.0 mg/kg (i. p.) and 0.5‐2.5 nmol (i. c.). Following drug delivery, the facilitatory effects of medial amygdaloid stimulation were blocked in a dose‐ and time‐dependent manner in which the effects were noted as early as 5 min postinjection. The maximum drug dose (4.0 mg/kg) employed for peripheral administration resulted in a 42% reduction in the facilitatory effects of the medical amygdala (P<0.002). This drug, when microinjected directly into medial hypothalamic defensive rage sites at the maximum dose level of 2.5 nmol, resulted in an 84% reduction of the suppressive effects of amygdaloid stimulation (P<0.5) at 5 min postinjection. In the next study, an N‐methyl‐D‐aspartate (NMDA) antagonist, DL‐α‐amino‐7‐phosphonoheptanoic acid (AP‐7), was administered either peripherally (0.1‐1.0 mg/kg) or intracerebrally (0.2 and 2.0 nmol) into PAG defensive rage sites. Facilitation of defensive rage behavior, which was observed following dual stimulation of the basal amygdala and PAG, was significantly reduced by either route of drug administration in a dose‐ and time‐dependent manner. At the maximum dose level of peripheral administration, AP‐7 reduced amygdaloid facilitation of defensive rage by 63% (P<0.001) for 60 min, postinjection. A smaller (i. e., 19%) but still significant (P<0.05) reduction in facilitation was obtained following intracerebral administration of the drug. In a third study, the non‐selective opioid receptor antagonist, naloxone (27.5 nmol), infused directly into PAG defensive rage sites, totally blocked the suppressive effects of central amygdaloid stimulation for a period of 30 min (P<0.05) in a dose‐ and time‐dependent manner.The anatomical phase of this study revealed the following relationships: 1) that large numbers of neurons projecting to the medial hypothalamus from the medial amygdala immunoreact positively for substance P; 2) that neurons projecting to the PAG from the basal complex of amygdala immunoreact positively for glutamate and aspartate; and 3) that neurons located within the central nucleus of the amygdala which project to the PAG immunoreact positively for met‐enkephalin. Collectively, these observations provide new evidence which characterizes the likely neurotransmitters linked with specific amygdaloid pathways subserving the mo