After the pioneering studies by Ussing and co-workers, studies of epithelial Nael transport have come a long way. The first phase of the phenomenological description of the cell as a black box has been follow­ ed by studies of cellular mechanisms, the interplay of the different trans­ port components, and the mechanisms of regulation. A broad spectrum of methods has been applied to many epithelia in a variety of species. For the individual epithelia transport schemes have been proposed, and, at this point I think it is appropriate to take a pause and search for elements common to several epithelia. This aspect triggered the publica­ tion of this book, and in fact the various chapters emphasize that the funetional eomponents, expressed in the various epithelia, are not in­ finite in number, but they occur in epithelia which are separated in evolu­ tion by several hundred million years. The authors come both from the field of veterinary and human physiology as weIl as from biology. In my opinion, the close contacts and eollaborations between physiologists and biologists have been essential for the progress in this field. I wish to thank all authors for their con­ tributions, and I hope that the reader will appreciate this collection of up to date reviews on epithelia in nonvertebrates and vertebrates.

1 NaCl Transport in Gills and Related Structures.- I: Invertebrates.- II: Vertebrates.- References to Parts I and II.- 2 Sodium and Chloride Transport Across the Invertebrate Intestine.- 1 Introduction.- 2 Transport Phenomena in Aplysia Intestine.- 3 Model for Chloride Absorption.- 4 Conclusions.- References.- 3 NaCl Transport in the Vertebrate Intestine.- 1 Introduction.- 2 Intestinal Water Transport.- 3 Intestinal Epithelium is a Leaky Epithelium.- 4 Experimental Approaches to Study Ion Transport by Intestinal Epithelium.- 5 Plasticity of Transport Functions in Intestinal Epithelium.- 6 The Equivalent Electrical Circuit: Spontaneous Short-Circuited Leaky Epithelia.- 7 Sodium and Chloride Absorption in the Small Intestine.- 8 Sodium and Chloride Absorption in the Colon.- 9 Intestinal Secretion.- References.- 4 NaCl Transport in the Kidney.- 1 Introduction.- 2 Segmental Organization of Renal NaCl Transport.- 3 Mechanisms of Renal NaCl Transport.- 4 Hormonal Modulation of Renal Tubular NaCl Transport.- References.- 5 NaCl Transport in Amphibian Skin.- 1 Introduction.- 2 Functional Organization of Amphibian Skin.- 3 The Transport Systems of the Membranes.- 4 Conclusions.- References.- 6 NaCl Transport in Tracheal Epithelium.- 1 Introduction.- 2 Relation of Sodium and Chloride Transport to Mucociliary Clearance.- 3 Transport Properties of Dog Tracheal Epithelium.- 4 Cellular Mechanism of Chloride Secretion.- 5 Cellular Mechanism of Sodium Absorption.- 6 Regulation of Chloride Secretion.- 7 Regulation of Sodium Absorption.- References.- 7 NaCl Transport in Salt Glands.- 1 Introduction.- 2 The Shark Rectal Gland.- 3 The Nasal Salt Gland of Birds.- 4 Salt Glands in Reptilia.- References.- 8 Regulation of NaCl Transport in Tight Epithelia.- 1 Introduction.- 2 Sites of Regulation.- 3 Regulation in the Time Frame of 0-10 s.- 4 Regulation in the Time Frame of 10 s to 10 min.- 5 Regulation in the Time Frame of 10 min to 1 h.- 6 Regulation in the Time Frame of 1-3 h.- 7 Regulation in the Time Frame of 3-24 h.- 8 Regulation in the Time Frame of 24 h.- 9 Summary.- References.