A sensitive radioimmunoassay for beta-endorphin is described. Antibodies against human beta-endorphin which exhibit a high avidity for the C-terminal of the peptide were raised in rabbits following the injection of thyroglobulin-coupled human beta-endorphin (betah-E) as immunogen. Methionine-enkephalin, alpha-, gamma-endorphine, as well as ACTH peptides did not cause interference in the radioimmunoassay. beta-Lipotropin, however, showed a 50% cross-reactivity. The sensitivity of the assay is 25 pg/0.5 ml tube volume for beta-endorphin. beta-Endorphin was extracted with a high recovery from the rat plasma using silicic acid and beta-endorphin levels as low as 100 pg/ml could be measured. Basal levels of beta-endorphin-like immunoreactivity in plasma of rats were about 400 pg/ml. beta-Endorphin levels in adrenalectomized rats and in animals chronically treated with the cortisol synthesis blocker metyrapone were found to be markedly increased (about 7-fold). Exposure of the rats to electrically induced footshocks caused a similar increase of immunoreactive beta-endorphin in plasma. A significant increase was also seen after insulin injection.