Secure communication guaranteeing reliability, authenticity, and privacy in sensor networks with active adversaries is a challenging research problem since asymmetric key cryptosystems are not suitable for sensor nodes with limited computation and communication capabilities. In most proposed secure communication protocols, sensor nodes need to contact the base station to get a session key first if two sensor nodes want to establish a secure communication channel (e.g., SPINS). In several environments, this may be impractical. In this paper, we study key agreement protocols for which two sensor nodes (who do not necessarily have a shared key from the key predistribution phase) could establish a secure communication channel against active adversaries (e.g., denial of service attacks) without the involvement of the base station.