Ad hoc and wireless sensor networks have recently emerged as successful technologies in a number of application domains. The need to build security services into them remains however a considerable challenge as the hardware used often shows serious processing and energy limitations. This work evaluates the impact of a number of security mechanisms on sensor nodes and the network as a whole. Hence a number of actual measurements were undertaken in a real sensor platform in order to accurately establish energy consumption for various encryption algorithms as well the baseline scenario obtained when none of these is used. Measurements have shown that integrity code length added to application messages using some cryptography algorithms and MAC (Message Authentication Code) is acceptable for a sensor node with 128 KB of ROM memory and 4 KB of RAM (MICA2). We also were able to check that power consumption of the encryption process does not in itself cause representative impact, since it is in the micro-joules range.