This paper describes testing of rule conditions and execution of rule actions in the ariel active DBMS. the ariel rule systems in tightly coupled with query and update processing. ariel rules can have conditions based on a mix of paterns, events, and transitions. for testing rule conditions, ariel makes use of a disrimination network composed of a special data structure for testing single-relation selection conditions efficently, and a modifed version of the TREAT algorithm,called A-TREAT, for testing join conditions. the key modification to TREAT (which could also be used in the rate algorithm) is the use of virtual a-memory nodes which save storage since they contain only the predicate associated with the memory node instead of copies of data matching the predicate. the rule-action executor in ariel binds the data matching a rule's condition to the action of the rule at rule fire time, and executes the rule action using the query processor.