Pituitary stalk lesions produce a triphasic disturbance in water metabolism. A short-lasting polyuria is followed by an oliguric period of 3 days'' duration, which then merges into a manifest diabetes insipidus. During the oliguric phase ADH [anti-diuretic hormone] content of the degenerating posterior lobe considerably declines. The activity of the pituitary-adrenal system during the 3 phases of the diabetes insipidus remains at a constant basal level. The system is unresponsive to noxious stimuli and only partially responsive to lysine vasopressin (LVP). During the oliguric interphase neither LVP nor a crude median eminence extract was capable of releasing ACTH [adrenocorticotropic hormone]. Since adrenal sensitivity to ACTH of rats with stalk lesions was normal and ACTH content of their anterior pituitaries, although temporarily diminished, was enough to stimulate the adrenals if released, the results are interpreted to indicate that the pituitary of rats with stalk lesions is at least temporarily isolated and rendered unresponsive to corticotropin releasers by damage to the portal vessel system.