Adequate electrical stimulation of extreme lateral hypothalamic regions of healthy, non-aggressive male cats was employed to produce aggression on live but anaesthetized rats. Stimulus response (S-R) curves based on scoring systems for both somatic and affective display components of behaviour were used to assess how manipulation of midbrain PAG by electrocoagulative lesions or drug microinjections affected the sensitivity of attack producing hypothalamic loci. Anodal lesions of dorsal PAG and adjoining tectum increased the excitability of hypothalamic loci producing predatory attack. Microinjection of 250 ng of delta-alanine-methionine enkephalin (DAME) in dPAG completely suppressed the somatomotor components of attack behaviour and markedly inhibited the affective display components. Administration of naloxone, an opioid antagonist (1 microgram) at the same sites facilitated the hypothalamically induced attack behaviour and annulled the inhibitory effect of DAME. These findings indicate the involvement of midbrain enkephalinergic mechanisms in the modulation of predatory attack behaviour elaborated by hypothalamic stimulation.