The life span of taste bud cells in rat circumvallate papillae was measured by autoradiography after labeling them with a pulse of [3H]thymidine. Specimens of circumvallate papillae were taken daily 1.5-18.5 days after the isotope was administered; thereafter, specimens were taken on alternate days until 25.5 days. For each time interval, the number of labeled cell nuclei was counted in 200-450 taste buds and plotted as the ratio of labeled cells/taste bud v. time faster injection of [3H]TdR. In all, 6958 taste buds were counted. The total number of labeled cells (dark plus light) per taste bud reached peaks at 6.5, 13.5 and 20.5 days. The curve for the number of labeled dark cells/bud had essentially the same shape as that for total cells. The number of labeled light cells/bud reached a modest peak at 6.5 days and slowly declined to a plateau for the remainder of the experiment. The data show that an average of 2 days elapsed after injection before labeled dark cells entered the bud and they spent an average of 7 days in the non-proliferating taste bud compartment; thus, the life span of the dark cell was 9 days. The life span of the light cell was difficult to estimate quantitatively, but this cell type was labeled at a much slower rate than dark cells and is assumed to have a signifcantly longer tenure in the taste bud.