Background Noninvasive brain stimulation is a powerful way to modify excitability of the cerebral cortex in humans and is increasingly used to treat psychiatric disorders. The observed clinical effects are in the moderate range and it has been suggested that the efficiency of brain stimulation depends on the underlying cortical state. Objective To isolate and manipulate brain rhythms associated with cortical excitability. Methods In the first experiment electroencephalography (EEG) and transcranial magnetic stimulation (TMS) were interleaved to study associations between brain oscillations and the amplitude of the motor evoked potential (MEP) during isometric contraction. Results of the first experiment were used in a second experiment to selectively modulate cortical excitability levels by applying transcranial alternating current stimulation (tACS). Results A linear regression showed that MEP amplitude could be modeled by θ (4-7 Hz) and β (13-30 Hz) oscillations recorded over the left and right M1. Significant increases in cortical excitability were found after θ (5 Hz)-β (20 Hz) tACS as compared with baseline and α (10 Hz) tACS. Conclusions Scalp-recorded brain oscillations can serve as a proxy for the effective modulation of cortical excitability by mimicking natural brain rhythms using weak electric currents.