This book presents the papers that were delivered at the Satellite Symposium of the International Society for Neurochemistry in Padua, September, 1975. Having such Satellite Symposia was a new experiment for the Society, and all signs, including those from Padua, indicate that it was a very successful experiment, which will be an old tradition for the Society. The large international meeting affords the opportunity for presentations from all areas, for meeting of colleagues from various backgrounds and disciplines. The Satellite Symposia allow people from the same area of interest to discuss their subject in depth, and as such represent meetings of possibly the greatest practical significance for the participants.

The Padua Symposium was no exception: all who participated in it could testify to its success in exchanging of information, learning of new approaches, and acquiring of new ideas, also the strengthening of old friendships, forming of new friendships and new collaborations, and exposing ideas to criticisms, suggestions, discussions. As a sign of the interest and success of the participants I can mention that all participants have sent in their contributions. Perhaps editors of volumes would appreciate that the very last came in no later than two months past the deadline. The symposium could not have been organized at a more suitable place than Padua --

for two days in the University, one of the oldest and richest in traditions, and a third day at the villa Simes under the most spacious, indeed princely circumstances. All these aspects, and the friendly hospitality that contributed so much to the cooperative spirit, would not have been possible without the help and efforts of the hosts, the organizers, and the participants. I have no doubt that this meeting will belong among the most pleasant memories and among the most productive and profitable times of the participants. Barriers of the brain, somewhat misnamed the "blood-brain barrier", have been with us for a long time, have been investigated often, and are considered in general among the most specific and prominent properties of the brain. In many ways, in spite of separating this organ from the rest of the organism and providing it with special properties and functions, it is more often misunderstood than understood and its complexity is rarely recognized.

It is more recently becoming increasingly clear, however, that the homeostatic equilibrium, and its physiological and pathological changes, are governed by a complex set of mechanisms residing heterogeneously in a complex set of membranes. Mechanisms residing in the capillary endothelium are of importance - especially in short term penetration studies of administered substances from blood to brain; the physiological and pathological equilibria are likely to be more influenced and governed by mechanisms residing in neuronal, glial, nuclear, and lysosomal membranes, to name a few. For functional importance in turn, mechanisms residing in mitochondrial and synaptosomal membranes are of crucial importance, for transport out of the system those present in choroid plexus among others have to be studied.

To focus all our interest on one membrane, or to believe that the distribution of substances in brain is governed by the properties of a single membrane would constitute ignoring the complexities of the nervous system - an obvious error. The present symposium focused its interest on the transport systems present in these various membranes, and showed the great deal of important information that has been obtained in the past few years in this area. The information discussed here, beyond summarizing our present knowledge, can serve to stimulate further advance in this important field. Membrane phenomena are a most important and exciting field in biology, and in few areas are they of greater interest than in studies of neural function. As in all such symposia, a number of potential contributors could not participate, but the possibilities of the field and the breadth of approach is clearly illustrated in this volume, which represents a rapidly developing field of investigation.