A novel adaptive and distributed fair scheduling (ADFS) MAC protocol for the regime of wireless sensor networks is presented. Besides data, a wireless sensor network may also transmit video and voice for surveillance and exploration applications. Since, fairness is important, the proposed ADFS protocol allocates the channel bandwidth in proportion to the weight of the packet flows. The weights are assigned initially according to the desired quality-of-service (QoS) and they are updated as a function of network conditions. The backoff interval is also altered to ensure that global fairness is attained even with variable service rates. Fairness and throughput bounds in the presence of channel variations and dynamic backoff intervals are shown analytically. The results demonstrate a better fairness index, significant increase in aggregate throughput, and smaller delay variations over the other available schemes like DFS and IEEE 802.11 protocol.