The content of immunoreactive (ir)-β-endorphin, ir-dynorphin 1–17, and ir-dynorphin 1–8 was determined in hypothalamus, anterior pituitary, and neurointermediate lobe of rats 3–24 months of age. In the anterior pituitary ir-β-endorphin showed a progressive rise with age (0.5 ± 0.06 ng/tissue at 3 months to 1.02 ± 0.23 at 24 months); a similar change was seen in ir-adrenocorticotropic hormone content of the same tissues. No age-related change in neurointermediate lobe ir-endorphin content was observed. In the hypothalamus, the ir-β-endorphin content fell progressively from 3 to 18 months, but was restored at 24 months to levels indistinguishable from those at 3 months (16.2 ± 4.3 ng/tissue compared with 11.3 ± 4.0 at 24 months). The ir-dynorphin content did not change progressively over the age span examined, except in the case of anterior pituitary content of ir-dynorphin 1–17 which fell progressively between 3 and 18 months (from 1.65 ± 0.15 ng/tissue at 3 months to 0.73 ± 0.12). Radioimmunoassay following high-performance liquid chromatography analysis of extracts of the tissues showed little variation of the immunoreactive forms with age, with two notable exceptions. In the hypothalamic extracts from 24-month-old rats the ratio of the nonacetylated ir-endorphin to the acetylated ir-endorphin was lower than in equivalent extracts from 3-month-old rats. In anterior pituitary extracts from 3-month-old rats, ir-dynorphin 1–17 appeared as two peaks (putative 6K and 4K species) of approximately equal size. Between 3 and 18 months the smaller form became predominant, so that in extracts from older rats the larger form was not present. These results indicate that aging influences the net synthesis-secretion balance of these opioid peptides differently, between ages and between tissues. Finally, posttranslational processing of some of these peptides differs with age, as indicated by changes in immunoreactive forms observed on high-performance liq