In MCF-7 cells, estrogen receptor (ER) elimination occurs rapidly under stimulation with estradiol (E2) at 1 nM ('ER processing'); cycloheximide (CHX) at 50 microM impedes this phenomenon. ER processing is also observed when E2 is removed after the first hour of incubation, indicating that the role of the hormone would be limited to the initiation of this process. When CHX is removed at the same time, receptor processing and, later, the induction of progesterone receptor (PgR) both proceed. The initial estrogenic signal which activates ER is therefore not influenced by CHX. In support of this conclusion, no effect of the drug on E2 binding affinity of residual ER was detected. A similar result was recorded for a series of estrogens and antiestrogens, indicating that CHX exerts no influence on the potential agonistic/antagonistic potency of any ligand. Size-exclusion chromatography (FPLC) revealed that [3H]E2-induced ER activation leads to the cleavage of the native receptor (67 kDa) into low molecular weight isoforms which subsequently become less detectable over time (proteolysis). In the presence of CHX, such ER isoforms persist, confirming the absence of interference of the drug with the activation step. When the cells were prelabelled with [3H]tamoxifen aziridine ([3H]TAZ) before their exposure to E2, ER cleavage could not be detected due to the lack of activation potency of the antiestrogenic ligand. However, the [3H]TAZ-ER complexes were subjected to E2-induced processing; CHX blocked this phenomenon, which is associated with the maintenance of ER synthesis and activation.