This series has been directed at providing scientists possessing considerable bio chemical background with specialized reviews of neurobiological interest. Some have dealt with completed bodies of research, while others consist of extensive reports of research in progress, judged to be of current interest to the active researcher. We have selected recognized scientists and allowed them freedom to reflect and speculate in the field in which they have achieved prom inence. We note with sadness the passing of Dr. 10rdi FoIch-Pi, who served as an advisory editor when the series was initiated. He played a central role in the development of neurochemistry, as well as the creation of professional societies and journals. He will be remembered fondly by all those whose lives he touched. The editors acknowledge the cooperation of the Upjohn Company in the preparation of the color plate included in this volume. We also acknowledge the skillful editorial assistance of Dr. Kenneth C. Leskawa. We are pleased to honor the retirement of Dr. E. Martin Gal, a former advisory editor of Advances, with the inclusion of a chapter by him in this volume.