A considerable body of imaging research has demonstrated morphological changes in the corpus callosum (CC) of patients with schizophrenia. Transcranial magnetic stimulation (TMS) allows the possibility for the in vivo investigation of a variety of aspects of brain function including the spread of information across the CC. We aimed to investigate whether patients with schizophrenia demonstrate abnormalities of transcallosal inhibition (TCI), a TMS parameter measured with both single and paired pulse experiments. 25 patients with DSM-IV schizophrenia and 20 normal volunteers participated in the study. Electromyographic (EMG) recordings from the bilateral abductor pollicis brevis (APB) muscle were made during focal TMS stimulation to the motor cortex. Experimental paradigms were utilised to measure both the timing and degree of the effect of TCI. The patient group demonstrated a reduction in the degree of TCI at rest and during a sustained muscle contraction. TCI commenced at the same time in the patient and the control group but was of prolonged duration in the patient group although the length of TCI correlated with medication dose. Patients with schizophrenia demonstrate a reduction in the degree of TCI that appeared independent of medication dose. The latency of TCI is not altered in the patient group suggesting that cortical inhibitory mechanisms, rather than corpus callosal ones, are likely to be the cause of these TCI alterations.