The present study examines whether two treatments known to induce refractoriness to exogenous morphine produce this desensitization through a change in the posttranslational processing of brain beta-endorphin (beta-End). The first experiment examined whether an ovarian steroid regimen which produces a transient desensitization of brain opiate receptor mechanisms alters beta-End processing in the preoptic area (POA), medial basal hypothalamus (MBH), and brainstem (BS). The second experiment monitored the effects of morphine pellet treatment, known to produce morphine dependency, on immunoreactive beta-End forms in the hypothalamus and periaquaductal gray area of the midbrain (PAG). The individual molecular forms of beta-End were separated using ion exchange chromatography and collection fractions were quantitated for beta-End immunoreactivity by RIA. The results show that regional differences occur in the posttranslational processing of beta-End. In the hypothalamus, MBH and POA, beta-End-(1-31) and its non-acetylated C-terminal cleavage products, beta-End-(1-27) and beta-End-(1-16) were the predominant forms of beta-End. The PAG pools produced a beta-End peptide elution profile similar to the hypothalamus, although small amounts of N-acetyl-beta-End-(1-31) were also identified. The BS exhibited the least posttranslational processing of beta-End; beta-End-(1-31) was the primary product with smaller amounts of beta-End-(1-27) and beta-End-(1-26) observed. However, neither ovarian steroid treatment nor chronic morphine produced any changes in posttranslational processing of beta-End or in total beta-End concentration in any of the brain regions examined in these experiments. These data indicate that the refractoriness or tolerance to exogenous morphine associated with steroid or chronic morphine treatment cannot be explained by alterations in the biological activity of beta-End resulting from the differential regulation of its posttranslational processing products.