We propose an opportunistic transmission strategy for wireless sensor networks (WSNs) that operate in a strict energy-constrained environment. Our proposed transmission strategy named binary-decision based transmission (BDT) attempts to transmit at good channel conditions while meeting the delay constraint for delay-sensitive applications and the buffer size constraint for non-delay-sensitive applications, under time- varying wireless channel. The Markov decision process (MDP) is used to find an optimum threshold for transmission decision. Our BDT scheme initiates transmission only when the channel quality exceeds the optimum threshold, so that unsuccessful transmissions causing a waste of energy are avoided whenever possible.