This paper presents the design, implementation and performance evaluation of a new MAC protocol, called A-MAC, for wireless sensor networks. A-MAC combines the strengths of TDMA and CSMA to achieve the goal of low power transmissions for long-term surveillance and monitoring applications, where sensor nodes are typically vigilant for a long time and inactive most of the time until some event is detected. A-MAC employs an advertisement mechanism to eliminate collisions and reduce the overhearing and idle listening, which are the major energy wastes in wireless sensor networks. The distinctive feature of A-MAC is that a node needs to be active only when necessary as it is the transmitter or the receiver. During other times it can safely turn off its radio. Furthermore, to meet different application requirements, A-MAC supports two operation modes by which nodes can adaptively switch their operation modes according to the instant requirements and conditions of the network. A-MAC is implemented in TinyOS. By comparing A-MAC with existing MAC protocols, we show that A-MAC presents significant improvements in terms of power consumption and throughput.