Many applications in wireless sensor networks (WSNs) demand security. The standart low-power wireless sensors (TmoteSky, MICA2) have severe resource constraints. Despite some work on the adaptation of public-key cryptography to the resource constrained sensors, it is still not obvious whether the resources on the sensors are sufficient for the current asymmetric algorithms run in decent time and space. Therefore, Symmetric cryptography is preferred in most wireless sensor network applications since its resource requirements are lower than the resource requirements of the public-key cryptography. Key management is challenging since sensors can land anywhere after deployment. Earlier approaches on key management mostly focus on key predistribution where a small number of keys are placed in sensors before deployment. In this paper, we propose key distribution schemes using a mobile element. In proposed schemes, mobile robot handles all the overload of key distribution requiring minimal resources at the sensors for key management. Mobile robot broadcasts key messages within a radius, and all the nodes within the area can receive and process the message. We investigate the proposed schemes in detail and show that they are feasible using simulations. Mobile based key distribution can provide a comprehensive key management framework since it can also aid in detection of compromised nodes and do key revocation.