Transcallosal inhibition can be studied in humans using transcranial magnetic stimulation (TMS) using two protocols: ipsilateral silent period (ISP) and interhemispheric inhibition (IHI). Recently, a paired-pulse protocol that normally produces short-interval intracortical inhibition (SICI) on corticospinal output was also shown to reduce the transcallosal inhibition as measured by ISP (Trompetto et al. 2004). This shows that transcallosal neurons are modulated by inhibitory interneurons in a similar way to corticospinal neurons. Tokimura et al. in 1996 described that paired pulses of equal intensities produce a 3-phase pattern of facilitation of the corticospinal pathway that is believed to be due to facilitatory indirect ‘I’ wave interaction. The aim of our study is to identify if a similar excitatory modulation of transcallosal neurons exists using ISP and IHI. Motor evoked potentials were recorded from both first dorsal interossei (FDI) muscles in 10 healthy subjects. For the ISP protocol, 2 stimuli of equal intensity were delivered at different interstimulus intervals (ISI) to the left primary motor area (M1) while the subjects were activating the left FDI. The ISIs used were 1.3ms, 1.5ms, 2.0ms, 2.5ms, 3.0ms, 3.3ms, 3.5ms and 4.3ms; and the intensity of stimulation was set at the threshold for evoking ISP. For the IHI protocol, subjects were at rest and 2 conditioning stimuli (separated by the same ISIs as above) were applied to the left M1 with a test stimulus to right M1 40ms later. The intensities of the conditioning stimuli were equal and set at the threshold for IHI. For both the protocols, we found that the MEP in the right FDI was facilitated at 1.5ms and 3.0ms (p<0.05). ISP area was also facilitated at the same ISI in the left FDI (p<0.05). IHI from left-to-right M1 was enhanced at these ISI (p<0.05). The increase of ISP area and the degree of IHI were both correlated with the degree of I-wave facilitation in the other hemisphere (p<0.05). This is compatible with the idea that neural circuits that generate I-wave inputs to corticospinal neurons in cortical layer V also exist for transcallosal pyramidal neurons in layer III.