The level of opioid peptides: beta-endorphin and dynorphin, binding to the mu and kappa opioid receptors and the analgesic response of those endogenous opioid systems to stress were investigated in two strains of mice: C57BL/6 (C57) and DBA/2 (DBA). The nociceptive threshold of DBA mice was higher than that of C57 mice. KD values for spinal mu receptors were lower in C57, while KD for cerebral kappa receptors were higher in this strain. DBA mice have significantly higher concentrations of dynorphin in the hypothalamus and neurointermediate lobe of the pituitary. Stress-induced analgesia was much greater in C57 than in DBA mice. In the hypothalamus both stress procedures depressed the concentrations of beta-endorphin in C57, and dynorphin in DBA mice. The level of beta-endorphin increased in the neurointermediate lobe in C57 and in anterior lobe of the pituitary in DBA mice. In the spinal cord both stress procedures depressed the dynorphin level. The above data indicate that C57 and DBA mice differ in the endogenous opioid peptide content, stress-induced alteration and opioid receptor affinity, the effects which might correlate with their different responses to environmental factors and pharmacological agents.