Developmental changes in the plasma insulin response to milk ingestion have been determined in neonatal lambs fed by bottle and in naturally suckled infant rabbits. In lambs at 4 days of age and older plasma insulin increased within 5 min of suckling and declined again before increasing to a second peak 60 to 120 min after the feed. The early insulin peak was not associated with any change in plasma glucose and was absent in lambs at 1 day of age. When the early insulin peak was absent the increase in plasma glucose following the meal was greater. The early, apparently reflex, release of insulin also occurred in some 'sham-fed' lambs. Atropine administration before feeding abolished the early peak in insulin. A marked early peak of insulin release, not accompanied by any consistent change in plasma glucose, was also observed in naturally-suckled infant rabbits at 1, 5, 10 and 15 days after birth.