6 healthy subjects were studied on 3 separate occasions after ingestion of an ordinary breakfast. Once the meal was given alone, once together with somatostatin infused i.v. (100 micrograms/h) for the first 2 h after start of the meal, and once the meal was preceded by a 34-h fasting period. The postprandial plasma PP secretion was completely inhibited during the somatostatin infusion, and significantly augmented (p = 0.032) after the prolonged fast. During the 34-h fasting period, the PP levels were almost unaffected. The same group of subjects were also studied during a "normal" day with 4 regular meals. Plasma PP was found to increase shortly after breakfast and did not return to basal levels before early the next morning. The integrated PP secretion during the 24-h period showed a significant (p less than 0.003) correlation with the basal levels.