ACTH release elicited by ether inhalation or ethervenesection and measured by changes in plasma corticosterone level was studied in rats at various time intervals after placing a complete or an anterolateral cut around the medial basal hypothalamus (MBH). There was no rise in ‘resting’ plasma corticosterone after acceptable isolations and, in 5 out of 6 series of experiments, the complete or long anterolateral cuts prevented the rise in plasma corticosterone normally induced by ether stress. In contrast, ethervenesection elicited a significant increment of plasma corticosterone when the basal region of the lateral retrochiasmatic area (RCAL) was not completely transected. Possible reasons why these experiments failed to confirm previous findings are discussed. These results support the hypothesis that corticoliberin-containing fibers enter the MBH from outside and that most of these fibers run through the RCAL on their way towards the neurohemal regions of the infundibulum.