A model which suggests that there is a common structural basis for the action of many different classes of CNS drugs is described. It is shown that this general model is consistent with specific models for opioid analgesic and antidepressant activity. The significance of these models is not only that they define specific spatial relationships between the structural requirements in different CNS drug classes, but also that they allow the formulation of three very simple drug design techniques which will be referred to as pruning, splicing and grafting. When combined with available structure-activity information, these techniques may provide a rational approach to the design of drugs with specified CNS activity.