How to ensure medium access priority for specific traffic in a fully-distributed ad hoc network is still a relatively open problem. However, for some applications such as emergency message dissemination in ad hoc networks, priority-ensured medium access is critical for timely message delivery. This paper proposes a new medium access control (MAC) scheme that provides priority-ensured medium access service for such applications in fully-distributed ad hoc networks. The basic idea of the scheme is to encode and deliver priority information by out-of-band pulses. The comprehensive simulation results in the paper show that the proposed MAC scheme is effective in serving emergency message dissemination in vehicular ad hoc networks.