Lower oesophageal pH was monitored in 270 dogs under anaesthesia. There were 47 episodes of gastro-oesophageal reflux (17.4 per cent), most of which occurred shortly after the induction of anaesthesia. The refluxate was usually acid (pH < 4.0), but in four of the episodes (8.5 per cent) it was alkaline (pH > 7.5). Gastric contents with a pH below 2.5 were refluxed on 27 occasions (10 per cent) for an average period of about 44 minutes. Regurgitation occurred in two of the dogs. Increased age seemed to be associated with an increased incidence of reflux and an increased gastric acidity. Body position (sternal, dorsal and left or right lateral) and the tilt of the body during surgery (horizontal or tilted to an 8 degrees head-up or head-down position) had no influence on the incidence of gastro-oesophageal reflux. Dogs undergoing intra-abdominal surgery had significantly more reflux episodes than dogs undergoing non-abdominal surgery.