A total of 216 male and female albino Wistar rats (Rattus norvegicus) between 10 and 60 days old from 42 litters were studied to determine mast cell characteristics within the thalamus. Whereas the mean numbers of mast cells within both the parenchyma and leptomeninges (combined) did not change appreciably after 15 days of age, the appearance of mast cells within the parenchyma (and disappearance from the leptomeninges) is clearly age-dependent. Between 15 and 25 days of age, the relative numbers of mast cells within the thalamic parenchyma changed from 20 to 90% of the total mast cells within thalamic space.