Ultrastructural changes in normal human prostate and benign prostatic hyperplasia (BPH) during long-term explant culture were compared. Explants of normal prostate obtained at immediate autopsy of young adults of BPH obtained at the time of surgery were maintained as long as 24 weeks in vitro. Ultrastructural changes occurring in epithelial cells during culture were monitored by transmission and scanning electron microscopy. Essentially identical results were found for normal prostate and BPH. During the first week of culture, secretory epithelial cells degenerated and sloughed into the acinar lumen, resulting in an accumulation of necrotic debris. During this period, however, epithelial cells with ultrastructural characteristics of basal cells remained viable, repopulated glandular structures, migrated from glands and ducts, and epithelialized adjacent cut surfaces, eventually covering the explant. On explant surfaces, these basal cells initially were squamous-like, but they later became typically cuboidal, polygonal, or sometimes columnar and formed an epithelium, two cells or more thick. Epithelium with similar features lined acini within explants. Epithelial cells at the surface or within explants were distinguished by the presence of microvilli, junctional complexes, multiple Golgi complexes, well-developed rough endoplasmic reticulum, polyribosomes, nuclei with prominent nucleoli, orthodox mitochondria, scattered tonofilaments, and a basal lamina. Some epithelial cells extended from the lumen to the basal lamina; others were oriented along the basal lamina and did not extend to the lumen. By 1--2 weeks in vitro, these epithelial cells began synthesis of mucus-like material. At later intervals of culture, microvilli were shortened and mucosubstances were reduced. During culture, the stroma became progressively hypocellular and necrotic. In summary, explant-cultured epithelial cells of normal human prostate or BPH were similar ultrastructurally and were found to originate from basal cells, which alone survive culture conditions.