Future application areas of wireless sensor networks (WSNs) may include industrial process automation, aircraft control systems or patient monitoring in hospitals. Such applications require predictable quality of service in terms of message transfer delay and reliability. Performance of WSN data transport is to a large extent defined by the employed medium access control (MAC) protocol. Currently, no existing MAC protocol is capable of supporting WSN applications that require deterministic data transport performance. In this paper, we present the implementation of a new WSN MAC protocol that is able to give deterministic bounds for message transfer delay and reliability. The protocol is implemented on TinyOS 2.x for a Tmote Sky node using the CC2420 transceiver. Our implementation shows that a deterministic MAC protocol with reasonable energy consumption patterns is practical.