—The IEEE 802.15.4 standard was developed for the purpose of media access control of low power wireless personal area networks. Wireless sensor network devices have the general characteristics of low power capabilities and operation. From this point of view, it makes technical sense to use the IEEE 802.15.4 standard for the MAC layer of wireless sensor networks. There are several modes of operation of the 802.15.4 protocol. In this study, we explore the use of non-beaconed mode for the operation of these networks. We modify this mode of the protocol in order to make it able to provide reliable delivery of high importance traffic within these networks, and at the same time ensure an energy-efficient operation of the network. We conduct simulation experiments to show the ability of our protocol to deliver highly important data traffic in a robust manner, while saving the energy resources of network nodes. Keywords-WSN; IEEE 802.15.4; energy-efficiency; reliability