An active real-time database system (ARTDBS) is designed to provide timely response to the critical situations that are defined on database states. Although a number of studies have already addressed various issues in ARTDBSs, little attention has been paid to scheduling transactions in a distributed ARTDBS environment. In this paper, 2 we describe a detailed performance model of a distributed ARTDBS and investigate various performance issues in time-cognizant transaction processing in ARTDBSs. The experiments conducted evaluate the performance under various types of active workload and different distributed transaction-processing architectures. The performance metric used in the evaluations is the fraction of transactions that violate their timing constraints. We also describe and evaluate a nested transaction execution scheme that improves the real-time performance under high levels of active workload. © 1998 Elsevier Science Inc. All rights reserved.