In 150 patients with undefined biliary pain after cholecystectomy, responses to morphine were compared with responses to morphine combined with neostigmine. The relationship between rises in plasma levels of aspartate aminotransferase (AST) after morphine or morphine-neostigmine and sphincter of Oddi motility as assessed by endoscopic manometry was also examined. When compared with morphine-neostigmine, patients given morphine alone showed a similar frequency (30% versus 33%) of increases in plasma levels of AST (greater than twice the upper limit of the reference range) but had less abdominal pain and a lower frequency of similar increases in plasma levels of amylase (4% versus 25%). Of 92 patients who consented to endoscopic manometry of the sphincter of Oddi, satisfactory manometric records were obtained in 84, 31 with and 53 without increases in AST after morphine or morphine-neostigmine. Those showing rises in AST had a higher frequency of abnormal manometric records (81% versus 57%, P = 0.025), higher basal pressures in the sphincter of Oddi (P = 0.0001) and higher pressures within ducts (P = 0.02). There was a significant correlation between sphincter basal pressures and intraduct pressures (r = 0.51, P less than 0.001). Rises in plasma AST after morphine are similar to those after morphine-neostigmine and are influenced by, or linked to, factors which determine sphincter basal pressures and intraduct pressures.