Primary cultures of normal rat prostate produce cells with epithelial characteristics that may be useful in the study of cell differentiation and prostate physiology. These epithelial cells, grown in DME plus 10% FCS, 10% HS, and 50 ng/ml each of hydrocortisone and insulin, can be induced to differentiate into dome ( hemicyst ) forming cells by 250 mM DMSO, a molecule that acts as a differentiation inducer in Friend erythroleukemia cells, Madin-Darby canine kidney cells, and in mouse and rat mammary epithelial cells. Dome formation in cell culture is the consequence of transepithelial transport of water and ions, resulting in fluid accumulation between the culture dish and the cell layer. Dome formation in prostate epithelial cells, besides representing an example of induction of differentiation in a cell culture system, suggests that prostate (ductal?) epithelial cells have fluid reabsorption capabilities.