BACKGROUND A canine model was used to define whether Nissen fundoplication inhibits gastro-oesophageal reflux by inhibiting transient lower oesophageal sphincter relaxations (TLOSR) or by creating a pressure barrier at the gastro-oesophageal junction. METHODS Four surgical models were studied pre-operatively and postoperatively. These were: (i) the surgical mobilization required for fundoplication (sham fundoplication, n = 5); (ii) a standard fundoplication (n = 4); (iii) anterior and posterior myotomy of the lower oesophageal sphincter (LOS; cardiomyotomy, n = 4); and (iv) combined cardiomyotomy and fundoplication (n = 4). Each operative procedure was assessed for its effect on the incidence of TLOSR and gas reflux events, the mean LOS pressure and the LOS pressure profile during swallow events. RESULTS Sham fundoplication reduced the rate of evoked TLOSR in response to gaseous gastric insufflation from 9.8+/-1.6/h (mean +/- SEM) to 5.4 +/-1.5/h. The mean LOS pressure was reduced from 25.1+/-2.6 to 18.5+/-2.1 mm Hg but nadir LOS pressure during swallowing was not altered. Nissen fundoplication virtually abolished evoked TLOSR from 10.4+/-1.2/h to 0.4+/-0.4/h, increased mean basal LOS pressure from 19.8+/-2.1 to 27.0+/-1.1 mm Hg and increased the nadir pressure on swallowing from 3.4+/-1.0 mm Hg to 14.4+/-1.0 mm Hg. Cardiomyotomy was associated with a near continuous leakage of gas across a chronically hypotensive LOS. Cardiomyotomy reduced the resting LOS pressure from 14.7+/-1.2 mm Hg to 2.3+/-1.0 mm Hg. Cardiomyotomy with fundoplication was associated with no loss of LOS competence. No gas venting episodes occurred either by passive leakage or by TLOSR. Cardiomyotomy with fundoplication was associated with a fall in mean LOS pressure from 14.3+/-1.5 mm Hg to 7.1+/-1.8 mm Hg but no LOS relaxation occurred during swallowing. CONCLUSIONS Nissen fundoplication is highly effective in preventing reflux across a normal or chronically hypotensive LOS. Fundoplication results in a constant, measurable pressure barrier at the lower end of the oesophagus that is not due to a change in intrinsic LOS tone. Following fundoplication TLOSR are prevented by the constant low-pressure barrier.