The glycine site on the NMDA receptor complex has generated an enormous amount of interest since it was first described five years ago. In this review by John Kemp and Paul Leeson the structure-activity relationships of agonists, partial agonists and antagonists acting at the glycine site are reviewed, along with what has been learned from studies with these compounds about the role of this site in physiological and pathological processes. Studies with prototype antagonists, and low-efficacy partial agonists that penetrate the brain, show that these compounds possess anticonvulsant and neuroprotective properties but lack some of the side-effects of other types of NMDA antagonists.