th This volume contains articles presented at the X International Symposium on Purines and Pyrimidines in Man, held on May 14 19, 2000 in Tel Aviv, Israel. The first symposium in this series took place in Tel Aviv in 1973. Since then, the symposium has been held every three years in different parts of the world, including Europe, USA and Japan. The participants, in this series of symposia, are characterised by a wide interest in the various aspects of purines and pyrimidines in man, which include biochemistry, genetics, pharmacology, physiology, clinics, etc. Presentations in the symposia include clarification of metabolic pathways, characterisation of enzyme structure and kinetics and discoveries of new inborn errors of metabolism and suggestions for new therapeutic approaches for these inborn errors. In addition, development of new purine and pyrimidine derivatives for the treatment of cancer and viral diseases, and many more subjects of mutual interest were brought to the fore. With the development of therapeutic means and of new research tools, we have witnessed changes in the areas of interest. The interest in gout and uric acid urolithiasis has lessened, whereas molecular aspects, the role of purine and pyrimidine substances in neurotransmission and in purinergic signaling appear to gain greater interest. The articles, included in this volume, contain new data pertaining to the various aspects detailed above.