We have studied the effects of vecuronium given in divided doses (priming principle) on the frequency of acid reflux into the oesophagus. Two groups of 16 patients were studied. After induction with thiopentone 6 mg kg-1 and fentanyl 3 micrograms kg-1, patients in one group were given vecuronium 0.01 mg kg-1 as a priming dose, followed by an intubation dose of 0.1 mg kg-1 4 min later. Patients in the other group were given no priming dose. Lower oesophageal pH was monitored continuously and acid reflux was defined as a decrease in pH to less than 4.0. One patient in each group had acid reflux during the time course of the induction. This study suggests that vecuronium 0.01 mg kg-1 given as a priming dose does not increase the risk of acid reflux into the oesophagus.