Taste bud cell turnover rate was examined in oral epithelium of the precocial chick, which at hatching contains the adult complement of taste buds. Forty newly hatched chicks received single or double pulse injections of tritiated thymidine (specific activity was 6.7 Curies/ millimole; dosage was 0.5 microcuriesig body weight, intraperitoneally). Anterior mandibular epithelium was processed for light microscopic autoradiography at 2 and 16 hours, as well as 1, 2, 3, 4, 6, 8, 10, 12, 14, 16, 18, and 20 days after the initial pulse. In a coded and randomized procedure, the section (7 pm) through the bud's center was selected for counting 2 6 silver grains over round-clear and gracile-dense gemmal cell nuclei. The mean number of labelled cells/bud varied significantly ( P I 0.01) during the first four posthatch days, yielding the fastest gemmal cell turnover rates (3.4-4.4 days) yet reported in vertebrates.Average bud diameter also significantly changed during the first four posthatch days, and was reflected in shifts of the distribution of 40-69 pm and 2 70 pm diameter buds. Both an increase in labelled bud cells and bud diameter during the first two posthatch days may reflect high proliferation rates in initially maturing buds. Subsequent decrease in bud diameter between 2 and 3 days postinjection may indicate splitting of large-diameter ( 2 70 pm) buds and/or normal bud cell death due to failure of sensory afferentation. Bud-splitting alone, however, cannot account for significant decreases in bud cell label which did not occur before 4-6 days postinjection. o 1994 Wiley-Liss, h e .Key words: autoradiography, birds, chemoreceptors, mouth floor, thymidine Gustatory receptor epithelium exhibits the capacity for cell renewal and thereby the potential to establish and replenish itself in normal development, as well as after denervation (e.g., Murray and Murray, 1971; Oliver and Whitehead, 19921, and in reinnervation and regeneration (see review, Ganchrow and Ganchrow, 1992). Yet in general, comprehensive studies in vertebrates provide limited evidence on the relation among turnover rates throughout an animal's normal lifespan. Whether turnover rates may reflect topographic (e.g., taste bud localization, distribution and density, and, frequency of tastant stimulation), trophic (e.g., the degree of sensory afferentation), or species-specific factors (see review, Ganchrow and Ganchrow, 1992) also has received scant attention.Studies of rodent bud cell renewal suggest that intragemma1 turnover rates may be age-, place-, or speciesdependent. In the rat, taste bud primordia first appear perinatally and the adult complement of circumvallate bud cells as well as final bud volume are achieved at 90 postnatal days (Hosley and Oakley, 1987). Circumvallate bud cells in weanling rats have a turnover rate of 7 days (Farbman, 1980), whereas those in adult mouse, 10.5 days (Conger and Wells, 1969). Fungiform bud cells in young adult rats are renewed in about 7.6 days (Beidler and Smallman, 1965).In contrast to gemmal develop...