With the purpose of investigating the mechanism of action behind the rise in gastro-oesophageal sphincter pressure induced by cholinergic activity, serum gastrin and sphincter pressure were recorded in 8 normal human subjects before and in relation to administration of carbachol. Carbachol was given subcutaneously in a dose of 4.5 mug per kg body weight. Mean sphincter pressure recorded 15, 30, and 45 minutes after administration of carbachol was significantly higher than the basal mean. The administration of carbachol did not lead to significant changes in serum gastrin and a relationship between serum gastrin and sphincter pressure was not demonstrable. It is concluded, that the carbachol-induced rise in the basal sphincter pressure is not dependent on an increase in serum gastrin, but probably attributable to either direct cholinergic stimulation of the receptors or a cholinergically enhanced sensitivity of the receptors for gastrin. Moreover, it is concluded that carbachol in individual dosages may be a suitable adjuvant in the medical treatment of patients with gastro-oesophageal reflux.