Transmission power control (TPC) is used in wireless networks to improve channel reuse and/or reduce energy consumption. It has been applied mainly to single-input single-output (SISO) systems, where each node is equipped with a single antenna. Multi-input multi-output (MIMO) systems can improve the throughput or the signal-to-noise ratio (SNR) by providing multiplexing or diversity gains, respectively. In this paper, we propose a power-controlled MAC protocol that combines different types of MIMO gains in a wireless network with two antennas per node. Our protocol, coined CMAC, allows for dynamic switching between diversity and multiplexing modes so as to maximize a utility function that depends on both the energy consumption and throughput. CMAC adapts the "antenna mode," the transmission power, and the modulation order on a per-packet basis. By "antenna mode" we mean one of the five possible transmit/receive antenna configurations: 1×1 (SISO), 2×1 (MISO-D), 1×2 (SIMO-D), 2×2 (MIMO-D), and 2×2 (MIMO-M), where the second, third, and fourth configurations offer a diversity (D) gain, whereas the last configuration offers a multiplexing (M) gain. By using control packets to bound the transmission power of potentially interfering terminals, CMAC allows for multiple interference limited transmissions to take place in the vicinity of a receiving terminal. We study via simulations the performance of CMAC in ad hoc topologies. Our results indicate that relative to nonadaptive protocols, CMAC achieves a significant improvement in both the overall energy consumption and the throughput.