Relay transmission is a promising technology for improving the throughput and energy efficiency in multi-rate wireless personal area networks (WPANs). In this paper, we propose a distributed relay MAC (DR-MAC) protocol in Wi-Media WPANs. DR-MAC extends a distributed reservation protocol (DRP) in WiMedia MAC and neighbor information for relay transmission can be collected during the beacon period. Therefore, DR-MAC can minimize control overhead for relay transmission and is compatible to the standard WiMedia MAC protocol. We also introduce a medium access slot (MAS) allocation procedure for maximizing the efficiency in DR-MAC. Compared with direct transmission, extensive simulation results demonstrate that DR-MAC can improve the throughput by 10% and reduce the energy consumption by 26% when the number of devices is 20.