We study the impact of spike-timing-dependent synaptic plasticity (STDP) on coherent gamma activity between distant cortical regions with reciprocal projections. Our simulation network consists of two areas and includes a STDP model reflecting efficacy suppression between pre/postsynaptic spike pairs as found in recent experiments during stimulation with spike trains (Froemke and Dan, 2002). We find that STDP in conjunction with common oscillatory input strengthens synapses with delays around multiples of the oscillation period. As a result, intrinsic excitatory interactions between the areas express in gamma waves with zero lag synchrony (instead of anti-phase synchrony without STDP). We discuss the impact of efficacy suppression on learning convergence and robustness.