We stimulated the motor cortex in 9 subjects using focal transcranial magnetic stimulation with a figure of 8 coil in order to examine the cortical representation of the erector spinae muscles. Recordings were made from the erector spinae 3.5 cm lateral to the third lumbar vertebra. In 5 subjects clearly reproducible responses could be obtained which had a latency compatible with transmission via fast conducting fibers in a mono- or oligosynaptic pathway. In the remaining 4 subjects responses were poorly defined. Latencies in surface recordings varied between 13 and 24 msec but were longer when needle recordings were used. Mapping of the motor cortex was performed by moving the coil in 2 cm steps on either side of Cz. Different patterns of hemispheric representation were found ranging from a contralateral projection in either hemisphere to a representation of both back muscles in one hemisphere (2 subjects). Responses were followed by a silent period. The latter was interrupted or terminated by a response between 52 and 85 msec post stimulus which was found predominantly in the muscle ipsilateral to the side of stimulation.