The third ventricular walls and floor of male and female mink (Mustela vison) were analyzed with comparative light microscopy, transmission and scanning electron microscopy. Distinct regional fine structural variations were noted in the appearance of the dorsal, middle third and lower portions of the ventricular wall and floor. These variations were characterized by an abrupt decrease in the density of ciliated ependymal surfaces in the middle one-third of the ventricular wall. The ventral walls and floor of the lateral and infundibular recesses displayed smoother contours with numerous bleb-like protrusions and blunt villiform processes. Occasionally multipolar neuron-like cells were observed to lie upon the floor of the infundibular recess. Speculation is raised relative to the differing functional capacities of these various ventricular areas.