Adult albino mice received tritiated thymidine either injected subcutaneously (333μCi) every 6 hours for up to 36 hours, or administered in their drinking water for up to 74 days (about 45μCi/day). Tissue sections were examined with autoradiography to determine the distribution of the label. Animals receiving subcutaneous injections showed no labeled cells in the choroid plexus epithelium and only one cell of 6600 cells counted in the ciliary epithelium was labeled. Animals exposed to the isotope in their drinking water for 74 days showed considerable uptake of label in hepatocytes (15%) and cortical kidney tubules (17%), whereas ciliary and choroid epithelia showed very low uptake at 0.25% and 0.01%, respectively. The data show that the ciliary and choroid epithelia have a very slow turnover rate compared to other slow renewing tissues, and, under normal conditions, these tissues are not renewed in the animal's adult life span.