Light microscope autoradiography was utilized to investigate the separate proliferative activities of the basal and glandular cell types during androgen-mediated restoration of the ventral prostatic epithelium in castrated rats. Our objective was to determine whether the proliferative population of the prostatic epithelium is restricted to the basal cell subtype under these experimental conditions. In the intact animal, substantial numbers of basal as well as glandular cells were found to be proliferating. The basal cell labeling index (LI) was 3.75% +/- 0.3% or 1.85 +/- 0.4 x 10(5) labeled cells per prostate. For the glandular cells the comparable figures were 0.38% and 3.32 +/- 1.5 x 10(5). Seven days after surgical castration, both subtypes persisted. However, only the numbers of glandular cells were found to be significantly reduced (66% decrease). When androgens were replaced by daily subcutaneous injection (testosterone propionate, 2 mg/0.2 ml sesame oil), both cell types responded with significantly increased proliferation. The temporal pattern of response displayed by each cell type was very similar. A greater proportion of basal than glandular cells responded, ie, 28% vs 11% LI. However, in terms of approximate numbers of cells labeled during the peak response, the value for the glandular cells was larger, ie, 2.4 vs 1.1 x 10(6) labeled cells. These data clearly indicate that under normal conditions as well as during testosterone stimulation, basal cells are not the sole population of proliferating cells in the prostatic epithelium. Thus, basal cells, as a distinct morphologic population, are not homogeneously undifferentiated stem cells which give rise to differentiated and nonproliferating glandular epithelial cells. Aspects of these data can nevertheless be interpreted to support the idea that the prostatic epithelium is maintained by a single cell lineage system in which the stem cells comprise a subpopulation of the basal cells. Alternatively, the two populations may be separate and distinct as no data are presented which unequivocally links the two cell types in a precursor-progeny relationship.