The ultrastructure of collecting duct epithelia was studied with the osmium impregnation technique in renal cortical explants grown in culture in the form of globular bodies. When this technique was applied to 7-day-old globular bodies, the endoplasmic reticulum (ER) of the superficial layer cells was faintly impregnated in the presence or absence of arginine-vasopressin (AVP) in the incubation medium; the ER of the cells located in the core of the globular bodies was densely impregnated with osmium. When these globular bodies were sectioned in 2 fragments and one was incubated in AVP for 30 min while the other was used as a control, a marked increase in osmium impregnation occurred: osmium black deposits were then noted in the lumen of the endoplasmic reticulum of two-thirds of the cells in the superficial layer. Various patterns of impregnation were observed. Cryptlike formations gave rise to mature epithelial cells showing the same pattern of osmium impregnation. When cyclic adenosine monophosphate (cAMP) was substituted for AVP in the incubation medium, the treated globular bodies revealed the same ultrastructural characteristics. Our data suggest that this primary culture of collecting duct epithelia is made up of heterogeneous cells with characteristics of principal and intercalated cells and that the AVP has a stimulatory effect on ER maturation, which is mediated by the adenylate cyclase system.