This paper presents an efficient distributed self-monitoring mechanism for a class of wireless sensor networks used for monitoring and surveillance. In these applications, it is important to monitor the health of the network of sensors itself for security reasons. This mechanism employs a novel two-phase timer scheme that exploits local coordination and active probing. Simulation results show that this method can achieve low false alarm probability without increasing the response delay. Under a stable environment analytical estimates are provided as a guideline in designing optimal parameter values. Under a changing, noisy environment a self-parameter tuning functionality is provided and examined. q 2005 Elsevier B.V. All rights reserved.