Primary somatosensory potentials (SEPs) were elicited by electrical stimulation of the medial sciatic (proprioceptive) or sural (cutaneous) nerves. They were detected by contacts on SI and MI dura on both cortical sides against a cephalic reference. SEPs were averaged (n = 20). Primary SI SEP consisted of positive, then negative, PI6/N30 waves. N30 was absent from MI records. Local electrocoagulation of the SI cortex on one side has entailed some reduction, but not suppression of together the homolateral MI and contralateral SI and MI SEP. The residual SEPs have increased in latencies by a few milliseconds. Additional coagulation of the MI area on the same side has resulted in loss of the SI and MI SEP on the opposite hemisphere when evoked by a stimulation ipsilateral to this intact cortex. Normal SEPs were elicited from the intact cortex by any of the used stimulation. No evoked signal could be evidence from the lesioned areas. It was concluded that negligible passive electrical diffusion from any SEP area was present onto any of the other SEP reception sites. From close comparison between the different records, we came to the following propositions: each of the SI and MI areas harbours a neural mass generator for SEPs elicited by contralateral nerve stimulation. SI and MI SEPs cannot be directly elicited by ipsilateral stimulus. SI and MI SEP ipsilateral to the nerve stimulation are due to some cortico-cortical trans-sagittal excitatory message arising from the contralateral SI/MI areas. Data stand for exteroceptive or proprioceptive stimulation as well. The absence of ipsilateral direct spino-cortical projection for SEP evidenced under barbiturate does also exist in the baboon after total recovery of surgery. A scheme is given which summarizes these active relationships between somesthetic areas.