This study was designed to investigate the proliferative activity of the buccal mucosa in healthy adult persons with regard to individual age. Punch biopsy specimens were taken from the buccal mucosa of 19 individuals (10 females, 9 males) ranging in age from 24 to 80 years, and were then examined autoradiographically, applying nuclear 3H-thymidine labelling in vitro under oxygen pressure of 2.2 atm. The mean total labelling index in the progenitor compartment amounted to 26.0 +/- 2.6% including 6.2 +/- 2.0% labelled basal nuclei and 19.8 +/- 2.6% labelled suprabasal nuclei. Related to 1,000 mu surface length, the mean number of basal cells runs up to 270.7 +/- 38.6 and the total nuclear labelling index to 70.9 +/- 10.4. This high rate of S phase nuclei, exceeding that of the epidermis several times, depends on the ample progenitor compartment as well as on the basal epithelium length which, as compared to the epithelium surface length, was shown to be extended by a mean proportion of 1.75. None of the autoradiographic results obtained in the present study revealed an age dependency of the buccal mucosa proliferative activity. The magnitude of the proliferative pool constitutes the matrix of the high renewal rate of the healthy buccal mucosa and provides a steady state of superficial cell loss and basal cell regeneration, that was proved, at least in the normal mucosa, to continue until senile age.