The effects of surgery, and of halothane and isoflurane, on oesophageal pressures were examined in 30 dogs. The dogs were premedicated with a combination of acepromazine maleate (0.1 mg/kg) and pethidine hydrochloride (1 mg/kg), and anaesthesia was induced with thiopentone (10 mg/kg). Ten of the dogs underwent abdominal surgery with halothane, 10 underwent abdominal surgery with isoflurane and 10 underwent non-abdominal surgery with halothane. Gastric pressure, lower oesophageal sphincter pressure and oesophageal barrier pressures were measured at five to 10 minutes after induction, five to 10 minutes after the initial surgical incision, during the abdominal surgery and while the skin was being sutured. There were no significant differences in lower oesophageal sphincter pressure between the groups of dogs but the pressures were greater in all the dogs during the surgical manipulation than shortly after induction or while the skin was being sutured. During abdominal surgery with halothane the barrier pressure was significantly higher (P < 0.05) than during non-abdominal surgery with halothane. The barrier pressure was lower (P < 0.001) during skin suturing after abdominal surgery with isoflurane than after abdominal or non-abdominal surgery with halothane. The lower oesophageal sphincter appears to be more sensitive to isoflurane than halothane.