Nineteen type I diabetics and 15 healthy subjects were given 500 ml of a mixed liquid test meal (flüssige Diabetikerkost, Fresenius, FRG) orally. Furthermore, heart rate variation during deep breathing and Valsalva maneuver was performed in order to test vagus nerve function. PP serum levels were determined before, and 2 to 160 minutes after finishing, the meal. In healthy subjects and in 10 diabetics without any sign of autonomic neuropathy of the vagus nerve a significant rise in PP serum values after the meal could be detected. In 7 type I diabetics with autonomic cardiac neuropathy (abnormal beat-to-beat variation during deep breathing and during the Valsalva maneuver) post-prandial PP levels remained low. Two diabetics without autonomic neuropathy were excluded from the test because endogenic PP antibodies were found in their serum. PP secretion after ingestion of a mixed protein-rich meal is mostly mediated by the vagus nerve or by the extravagal cholinergic system. After vagotomy, no PP secretion after a mixed meal could be detected. Measuring PP serum levels in diabetics after a mixed meal can be useful to check vagus nerve function in the gastrointestinal tract in order to detect autonomic neuropathy.