Between the 'permanent' cortex and medulla of the mouse adrenal there is a transitory cortex variously called the X, interlocking, boundary, or androgenic zone. At the height of its development the zone occupies about half the total width of the cortex. Its development, histology and variations in intact animals have been described elsewhere [Masui & Tamura, 1926; Tamura, 1926; Howard, 1927, 1930, 1937; Deanesly, 1928, 1938; Whitehead, 1933; Waring, 1935; Waring & Scott, 1937]. Sections of glands from a mouse 4 weeks old, of either sex, show a well-defined X-zone of highly eosinophil cells interlocking with the medulla. In the intact male, rapid degeneration of the zone begins at 28–30 days and leaves a connective tissue band between the medulla and permanent cortex. In males castrated before puberty, the zone does not degenerate at the usual time but increases in size. Degeneration does eventually occur in old castrated males.