A wealth of information on osmotic and ionic reaulation in Estuarine and Marine Animals has been accumulated over the past decades. Beyond early studies of whole-animal responses to changes in envi- ronmental salinities, efforts have been made later on to identify, to localize and to characterize the organs and structures responsible for the control of the characteristics of the cell's environmental fluid. When considering the problem of cell volume control in animals facing media of fluctuating salinities, we are indeed dealing with two different categories of mechanisms. A first one is concerned with the control of the osmolality of the intracellular fluid, hence with the processes directly implicated in the maintenance of cell volume and shape. They have been extensively described in several recent review papers. The second category includes the processes controllin the charac- teristics of the cell's environmental fluid in order to minimize the amplitude of the osmotic shocks the cells may have to cope with upon acclimation to media of changed salinities. They are localized in particular organs and structures: the so-called "caZt-transporting" epithelia. Up to now, most of the studies on salt-transportino epithe- lia in estuarine and marine animals used the black box approach, so that little or sometimes nothing is still known on the physiological, the biochemical and the biophysical basis of the transporting mecha- nisms as well as on the structure-function relationships.