In monogastric animals, suckling influences the secretion of gastrointestinal hormones during lactation. The aim of the present study was to investigate whether similar effects are induced by milking in cows. Experiments were performed on four cows in midlactation. Blood samples were drawn from a chronic jugular vein catheter and gastrin, and somatostatin were determined by radioimmunoassay. Milking and feeding increased plasma gastrin. Somatostatin increased at morning milking and at feeding, but it decreased at evening milking. Atropine injected subcutaneously 30 min before milking increased resting concentrations of gastrin but decreased resting concentrations of somatostatin. Feeding-induced release of gastrin remained but the milking-induced release disappeared. The milking- and feeding-induced effect on somatostatin became more marked. We suggest that milking influences gastrin and somatostatin via activation of the vagal nerves. The gastrin release caused by milking may be mediated via a cholinergic mechanism, whereas the atropine resistant effect on gastrin caused by feeding and on somatostatin caused by both milking and feeding suggest that a noncholinergic, perhaps peptidergic, transmitter may be involved.