Research on purine derivatives as potent and selective modulators of physiologic functions has moved to center stage. This volume covers the biology and chemistry of purines and of their receptors broadly under these section headings: - Adenosine Receptors and Effector Systems - Adenine Nucleotide Receptors and Effector Systems - Phosphodiesterase Enzymes and Inhibitors The presentations focus on the potential development of clinically useful drugs and powerful agents that activate or block purinergic receptors or that inhibit phosphodiesterases. Xanthines (caffeine and theophylline) represent one class of such agents. This volume is for pharmacologists, biochemists, and medical chemists in research labs of universities, gov- ernment, and the pharmaceutical industry.