We have summarized the quantitative relations developed so far for the description of NMDA receptor function. One of the most important gaps in our knowledge relates to desensitization. A full quantitative treatment of computational uses of NMDA receptor channels must await a formalization of this process and also a more detailed examination of the occupation of closed states of the receptor whose binding sites are occupied. As this information becomes available and the role of NMDA receptors in the function of brain circuits is further explored, we should be able to define accurately this second computational mode.