To study the regulation of nociceptive sensitivity/behavioral reactivity in animals during aversive states of different nature, the changes of vocalization thresholds and tail-withdrawal latencies were investigated in rats in free behavior, during restraint stress, after acute trauma to an extremity and under intraperitoneal acetic acid administration. To understand opioid peptide involvement in mediation of the changes obtained, this analysis was also done during opiate receptor blockade by naloxone. The data on the modification of vocalization and movement reactivity as well as on the changes of suprarenal weight and gastric ulceration, produced in normal and naloxone-treated rats by innoxious stressogenic, noxious somatic and visceral stimulation are discussed in relation with: 1. the peculiarities of sensitivity and responsivity of animals to external stimuli in aversive environment; 2. the role of these changes in maintenance of an animal's adaptive activity produced by environmental threat and their mediation by endogenous opioids; 3. the functional significance of the activation of endogenous opioidergic neurotransmission in organization, realization and modification of an animal's adaptive activity, directed on behavioral escape from aversive environment as well as on satisfaction of actual biological and zoosocial needs, in regulation of precise conformity among homeostasis, behavior and variable environment.