The present study investigated the way that sucking of a pacifier influences gastric secretory and motor functions in connection with tube feeding. Experiments were performed on eight preterm infants who were tube fed twice--once with and once without sucking of a pacifier. The time for tube feeding was significantly decreased and gastric retention decreased in five of seven infants when sucking a pacifier. Maternal milk was found to contain gastrin-17, somatostatin-14, and a somatostatin-like peptide larger than somatostatin-28. Somatostatin levels were significantly reduced in connection with non-nutritive sucking. Gastrin levels were increased in six of ten experiments 2 h and/or 3 h after bolus feeding, suggesting that these peptides were not only supplied by the milk, but were also released from the gastric mucosa. The presence of gastrin and somatostatin in gastric aspirates was established by use of chromatographic methods. The results indicate that somatostatin and gastrin are released into the gastric lumen in preterm infants and that sucking of a pacifier, in connection with bolus feeding, stimulates the gastric motor functions and facilitates the digestion process, probably via activation of vagal mechanisms.