This paper presents a performance analysis of adaptive modulation (AM) schemes in multiple-input multiple-output (MIMO) orthogonal space-time block code (OSTBC) systems over Nakagami-M keyhole channels. We consider a constantpower variable discrete-rate AM strategy and assume that the maximum ratio combining (MRC) technique is used at the receiver. Analytical expressions for the average spectral efficiency (SE), the bit error rate (BER), and the outage probability (OP), are derived and compared to the Monte Carlo (MC) simulation results. Numerical results illustrate the impact of the keyhole on the AM scheme performance and show a good agreement between the theoretical analysis and the MC simulations.