Gerhard Breves Department of Physiology, School of Veterinary Medicine, Hannover, Germany Title Endogenous circulation of phosphate in ruminants Abstract Ruminants are characterized by the endogenous circulation of phosphate (Pi) which is mediated by salivary Pi secretion and intestinal Pi absorption. The physiological relevance has to be regarded with respect to providing an additional buffer component for rumen pH as well as maintaining an adequate P supply for microbial growth. This could be confirmed by P depletion studies which have shown that both, rate of microbial fermentation and microbial protein synthesis was significantly reduced in response to P depletion. The cellular basis of endogenous Pi circulation has been studied in detail in recent years. From these studies it could be demonstrated that different epithelial mechanisms are involved in Pi secretion and absorption. Whereas NaPi type II transporters are involved in salivary Pi secretion different mechanisms have been identified for duodenal and jejunal epithelial tissues. In the duodenum of goats a H+-dependent transport system has been detected which can be modulated by Na+ and which does not respond to dietary P depletion whereas in the jejunum a NaPi type IIb transporter could be identified which is sensitive to H+ and which can be substantially up-regulated in response to P depletion. The up-regulation does not depend on respective activation of the Vit. D hormone system as it has been shown for monogastric species.   CV Gerhard Breves (56) has studied Veterinary Medicine in Hannover and was educated as a Physiologist. In 1990 he was appointed as Professor of Physiology at the University of Giessen and in 1997 he was appointed as Professor of Physiology at the School of Veterinary Medicine, Hannover. His main research area is gastrointestinal physiology with special emphasis to epithelial transport including functional and molecular aspects and mode of action of different feed additives such as pro- and prebiotics in the gastrointestinal tract.