Twenty-four hour pH monitoring is used increasingly as a technique for diagnosis of gastro-esophageal reflux (GER). The Synectics recording apparatus (Stockholm, Sweden) was used in 27 ambulatory control subjects in order to: identify the variations of the data-dependent factors (electrodes, buffer solutions, calibration, placement of esophageal electrode). The consequences of these variations were evaluated by studying the measurement deflection after 24 h, baseline variations, and a comparative double pH esophageal recording with reference equipment for 3 h postprandial; to provide normal values for several parameters of GER measurement under the conditions of this study (total number of reflux, number of reflux episodes longer than 5 min, duration of the longest reflux, percentage of time during which pH was less than 4.0). Important interindividual variations were observed. When interpreting the results provided by this type of apparatus, one should take into account these large variations due, first, to the technique used and second, to the conditions of ambulatory recording.