In recent years, the considerable progress of miniaturization and the consequent increase of the efficiency of digital circuits has allowed a great diffusion of the wireless sensor network technology. This has led to the growth of applications and protocols for applying these networks to several scenarios, such as the military one, where it is essential to deploy security protocols in order to prevent opponents from accessing the information exchanged among sensor nodes. This paper analyzes security issues of data processed by the WSN and describes a system able to generate sequences of random numbers, which can be used by security algorithms and protocols. The proposed True Random Number Generator (TRNG) exploits measurements obtained from sensor nodes, in order to allow every node to produce random data upon request, without involving a trusted third party. The proposed TRNG behavior has been tested by carrying out the NIST tests, and the obtained experimental results indicate the high degree of randomness of the produced numbers.