Background -Electrophrenic pacing can be used in the management of ventilatory failure in quadriplegic patients. A study was undertaken to determine the pattern oftransdiaphragmatic pressure (PDI) during the conditioning phase of electrophrenic pacing to see if it had a possible role in optimising the process of conditioning. Methods -The tidal volume (TV) and PDI were measured in a group of six quadriplegic patients commencing ventilation by low frequency pulse stimulation (7-10 Hz) and low respiratory rate stimulation (<10 breaths/min). Results -Tidal volume increased between baseline and month 1 (4.33 mllkg, p<0001) and between months 1 and 2 (3.00 mllkg, p<005) and then stabilised. PDI was higher during bilateral diaphragmatic pacing (mean (SD) 1-73 (0.30) kPa) than with either left (1 15 (0.34) kPa) or right (0.86 (0.37) kPa) unilateral pacing. PDI varied throughout the observation period, probably by interaction between recovery of the diaphragmatic fibres and the pacing regimen. Conclusions -Patients with quadriplegia due to high spinal injury can be maintained with ventilation by continuous electrophrenic pacing. The control criteria used in this study for pacing were tidal volume and the patient's tolerance, and the PDI measurement did not contribute any additional information to help with managing the conditioning process. (Thorax 1996;51:420-423)