Plasma somatostatin was followed for 24 h in 6 healthy young males having 4 regular meals with water allowed freely, and performing their normal daily activities. Plasma somatostatin rose significantly after each meal, it returned towards the basal level before the next meal, and it rose higher and stayed significantly elevated longer after the evening meal than after any of the other meals. A possible physiological circadian rhythm of plasma somatostatin is discussed.