Transmission power control (TPC) has been used in wireless ad hoc networks to improve channel reuse and/or reduce energy consumption. It has been mainly applied to single-input single-output (SISO) systems, in which each node is equipped with a single antenna. In this paper, we study MAC protocols for mobile ad hoc networks (MANETs) with MIMO (multi-input multi-output) capability. In particular, we consider the integration of MIMO into two MAC protocols. The first protocol is the IEEE 802.11 standard, which is a conservative protocol that does not use TPC. The second protocol is POWMAC, which exploits TPC to maximize the perceived throughput. We refer to these protocols when applied to MIMO systems as MIMO-802.11 and MIMO-POWMAC. We compare the performance of MIMO-802.11 and MIMO-POWMAC with SISO-802.11 and SISO-POWMAC, in terms of the perceived throughput and energy consumption. Our simulations reveal that although the MIMO system doubles the bit rate per link over the SISO system, the network throughput is not necessarily doubled due a reduction in the number of concurrent transmissions. In addition, the throughput gains in the MIMO system come at a non-negligible energy cost.