Teleosts do not possess uteri and ovulation occurs into the ovarian lumen. Hence, osteichthyan viviparity is characterized by intrafollicular gestation or intraluminal gestation (within the ovarian cavity), and thus, the ovary functions both in egg production and maintenance of the developing embryos. Because there is no attachment between embryonic and maternal tissues, maternal-embryonic exchange of gas, electrolytes, nutrients, metabolic waste, and possibly effector molecules and immunoglobulins is mediated by embryotrophe. In several instances it has been demonstrated that embryotrophic proteins originate from maternal serum rather than from elaboration within ovarian tissues. In intraluminal gestation, the ovarian wall becomes hypervascularized and the inner ovarian epithelium lining the ovarian cavity secretes embryotrophe. Trophotaeniae provide extraembryonic endodermal exchange surfaces in many species undergoing intraluminal gestation. There is a strong indication of a trophotaenial membrane binding system for proteins containing appreciable amounts of lysine. In intrafollicular gestation, yolk, pericardial and coelomic sacs play a major role in physiologic exchange. In most instances, intrafollicular gestation is associated with extraembryonic fine structural modifications consistent with respiratory and osmoregulatory functions. The postulated role in nutrient transfer requires experimental evidence. In matrotrophic viviparous teleosts, the gut may provide an effective absorptive surface. On the other hand, the skin, and perhaps the gills and buccopharyngeal tissue, serve as respiratory surfaces. As yet, no experimental evidence suggests exchange of metabolites at these sites. © 1993 Wiley-Liss, Inc.