The structure and distribution of tanycytes within the ependymal lining of the third cerebral ventricle of young (3 week old) and adult (24 week old) rats was examined using a modified Golgi-silver method. In both age groups, 3 tanycyte forms were identified on the basis of differences in the morphology of their basal processes: Type 1 - with long (greater than or equal to 200 microns), non-spinous basal processes; Type 2 - with long (greater than 100 microns) basal processes, the proximal segment of which bear spine-like projections and Type 3 - with short, spinous basal processes. In addition, tanycytes of 3 week old rats exhibited features that distinguished them from their respective adult forms. In these rats: i) tanycyte somata are smaller, more regular in outline and often bear apical processes, ii) the basal processes of Type 1 and 2 tanycytes are smaller in diameter and are less tortuous in their course through the neuropil, iii) the terminal segments of many Type 1 tanycytes bear spiny projections and iv) the proximal segments of Type 2 tanycytes are less coarse in appearance and bear distinctive fine projections. Differences in the regional distribution of tanycytic forms within each age group are also provided. Collectively, these observations suggest that tanycytes in 3 week old rats have not fully acquired adult form and possibly function.