In vitro and in vivo perfusion techniques were used to examine the changes in the release of β-endorphin, methionine-enkephalin (met-enkephalin), dynorphin and luteinizing hormone releasing hormone (LHRH) in response to the corticotropin releasing factor (CRF) receptor antagonist, α-helical CRF9–41. All four peptides were measured in the same sample collected at each time interval by specific radioimmunoassay methods. In vitro release experiments were conducted using slices of hypothalami obtained from male rats whereas the in vivo release of these peptides was assessed in push-pull perfusates of the arcuate-median eminence (ARC-ME) region of the medial basal hypothalamus of chloral hydrate-anaesthetized male rats. Treatment of rat hypothalamic slices in vitro with α-helical CRF9–41 (10–6M) resulted in a significant suppression of the release of β-endorphin and met-enkephalin within 10 min of application of the antagonist and a coincident significant increase in the release of LHRH. The levels of dynorphin were reduced but these changes were not significant. Within 10 min of withdrawal of the receptor antagonist and perfusion with normal (antagonist-free) medium the levels of these peptides returned to pretreatment values, i.e. the levels of β-endorphin, met-enkephalin and dynorphin rose while those of LHRH fell. Comparable results were obtained in vivo during push-pull perfusion of the ARC-ME region with α-helical CRF9–41. These concomitant temporal changes in the release of opioid peptides and LHRH in response to the receptor antagonist of CRF support the view that one of the mechanisms by which CRF inhibits the secretion of LHRH is by its stimulation of the release of opioid peptides within the brain. The data further suggest that the activity of opioidergic and LHRH neurons may be continuously under the influence of e