Nociceptive responses were evoked in cats by electrical transcutaneous stimulation of the forepaw or electrical stimulation of respective brain structures; these responses could be modulated (intensified or suppressed) by combined electrical stimulation of different brain structures or by neurochemical influences upon these structures. Intensification of nociceptive responses was observed after stimulation of the noradrenergic or P-ergic systems localized in the ventral zone of the central gray (vl SGC) and the structures monosynaptically connected with the latter: the posterior and lateral hypothalamic nuclei (Hp and Hl) and preoptic region (RPO). Similar effects were induced by suppression of the serotoninergic system concentrated within the dorsolateral central gray (dl SGC), dorsal raphe nucleus (Rd), and closely related structures: the ventromedial, dorsomedial, and paraventricular hypothalamic nuclei (Hvm, Hdm, and Hpv), septum (Sep), basolateral amygdalar nucleus (Am bl), fields 3–4 of the hippocampus (CA3–4), and cingular cortex (GC). Suppression of the serotoninergic system resulted in a decrease in the levels of functioning of the met-enkephalin- and β-endorphinergic systems and facilitation of the P-ergic system. Moderation of nociceptive responses, i.e., an analgesic effect, was observed after either stimulation of the serotonin-, met-enkephalin-, and β-endorphinergic systems localized in the dl SGC, Rd, Hvm, Hdm, Sep, Am bl, CA3–4, and GC, or suppression of the noradrenergic system. The latter influence resulted in inhibition of the P-ergic system and a rise in the functional activity of the met-enkephalin- and β-endorphinergic systems. The composition of two antagonistic brain systems, nociceptive and antinociceptive, is considered. The antinociceptive system includes serotonin-, met-enkephalin-, and β-endorphinergic elements. Leu-enkephalin is a nonspecific activator of the met-enkephalin-, β-endorphin-, and P-ergic systems. The nociceptive system consists of the vl SGC, Hp, Hl, and RPO, while the antinociceptive system includes the dl SGC, Rd, Hvm, Hdm, Hpv, Sep, Am dl, CA3–4, and GC.