To establish whether secretion of proglucagon-derived peptides (PGDPs) by the intestinal L cell is nutrient- and/or location-dependent, 0.9% saline, 200 mM glucose, or emulsified fats were administered into the ileal or duodenal lumen of normal rats. Fat administration, but not saline or glucose treatment, significantly increased circulating levels of the intestinal PGDPs in a time-dependent fashion, indicating a selectivity of the L cell in its response to nutrients. Interestingly, the response to duodenal fats was quantitatively and qualitatively identical to the response to ileal fats, despite 50-fold lower concentrations of PGDPs in the duodenum. These results suggest the existence of a duodenal factor that stimulates ileal PGDP secretion in response to fat ingestion. Ileal and plasma levels of gut PGDPs have been reported to be elevated in poorly controlled streptozotocin-diabetic rats. Whether the sensitivity of the L cell to luminal nutrients is altered in diabetes was, therefore, also examined. The L cell responses to luminal nutrients in diabetic rats were not significantly different from those of the normal rat, indicating a normal responsiveness of the L cell. However, independent of changes in glycemia, luminal glucose perfusion significantly decreased circulating glucagon levels in normal rats, but not in diabetics. Furthermore, luminal fat administration increased plasma glucagon levels in normal rats only. These results indicate that moderately controlled diabetes is associated with alterations in the pancreatic A cell, but not the intestinal L cell response to ingested nutrients. The results of the present study indicate that the response of the intestinal L cell to ingested food is nutrient-specific and that this specificity is not altered in diabetes. A duodenal-ileal axis is proposed to contribute to increments in circulating intestinal PGDPs in response to nutrient ingestion.