Although many studies have demonstrated that amygdala is involved in the antinociception, the mechanism has not yet been clarified. Previous studies using rats demonstrated that brief electrical stimulation to the amygdala depressed neural discharge in the cingulate areas while noxious stimulation was being applied to peripheral tissues. We investigated the distribution and the quantitative changes of endogenous opioids secreted in the periaqueductal gray (PAG) when electrical stimulation was applied to the amygdala. Male wistar rats (250-320g) were anesthetised with pentobarbital sodium (55-95mg/kg, i.p) and placed in a stereotaxic frame. Microelectrode was inserted into the central nucleus of the right amygdala and electrical stimulation (2μA, 100Hz, 15seconds) was applied (n=5). After 60 minutes, the rats were perfused with normal saline followed by 4% paraformaldehyde. The brain was removed, and cut into 20μm-thick transverse sections using cryostat. Untreated rats were used as controls (n=5). Some sections were stained with hematoxylin-eosin to confirm the stimulated area. The distribution and the quantitative changes of endogenous opioids (β-endorphin, enkephalin, dynorphin) secreted in the PAG were investigated immunohistologically. Immunopositive amount in a square of 100μm was determined by using appropriate software (Adobe Photoshop CS3 Extended, Adobe Systems Incorporated, USA). Statistical difference between stimulated and control rats was ascertained by Mann-Whitney U test. Conspicuous amount of β-endorphin, enkephalin and dynorphin was observed in PAG. Dynorphin was significantly increased in the entire area of the PAG in the stimulated rats (stimulated rats, immunopositive area of 140.63±112.04μm2 (mean ± S.D.) out of the 100μm square, and controls, 72.45±76.53μm2, p<0.05). Furthermore, dynorphin was significantly increased both in ipsilateral and in contralateral side in the lateral PAG when electrical stimulation was applied only to the right amygdala (ipsilateral side : stimulated rats, 143.66±107.22μm2, and controls, 56.90±32.28μm2, p<0.005 ; contralateral side : stimulated rats, 152.53 ±84.86μm2, and controls, 66.70±44.52μm2, p<0.005). Enkephalin in the stimulated rats increased, although not significantly, in dorsomedial and lateral PAG. Thus, secretion of the endogenous opioids in the PAG was increased bilaterally by applying electrical stimulation to the amygdala. These results suggest that electrical stimulation to the amygdala facilitates secretion of endogenous opioids in the PAG and leads to the facilitation of descending antinociceptive system.