Objectives: Evidence shows that repetitive transcranial magnetic stimulation (rTMS) changes cortical inhibition (CI) and excitability and that these changes may relate to its therapeutic effects. This study aimed to investigate the effects of differing durations or 'doses' of rTMS on cortical inhibition and excitability in healthy subjects. Methods: Four different experiments were conducted: 1 session of 1200. pulses of 1 or 20. Hz active or sham rTMS; 10 sessions of 1 or 20. Hz active or sham rTMS, 1200. pulses/session; 1 session of 3600. pulses of 1 or 20. Hz active or sham rTMS; 1 session of 6000. pulses of 20. Hz active or sham rTMS. Measures of cortical inhibition and excitability included short-interval intracortical inhibition, long interval cortical inhibition, cortical silent period (CSP), motor evoked potential amplitude, resting motor threshold and intracortical facilitation. Results: Only 6000. pulses of 20. Hz rTMS lead to a significant lengthening of the CSP and therefore potentiation of CI. There were no changes to excitability measures. Conclusion: Only high frequency rTMS potentiated CI. Longer treatment durations are required to produce such changes. Significance: Studies investigating the therapeutic effects of rTMS may benefit from extended dosing with increased number of pulses per session. CSP lengthening may be used to guide treatment response. © 2013 International Federation of Clinical Neurophysiology.