The human gene RSC1A1 codes for a 67 kDa-protein named RS1 that mediates transcriptional and posttranscriptional regulation of Na⁺-D-glucose cotransporter SGLT1. The posttranscriptional regulation occurs at the trans-Golgi network (TGN). We identified two tripeptides in human RS1 (GlnCysPro [QCP] and GlnSerPro [QSP]) that induce posttranscriptional down-regulation of SGLT1 at the TGN leading to 40-50% reduction of SGLT1 in plasma membrane. For effective intracellular concentrations IC₅₀ values of 2.0 nM (QCP) and 0.16 nM (QSP) were estimated. Down-regulation of SGLT1 by tripeptides was attenuated by intracellular monosaccharides including non-metabolized methyl--D-glucopyranoside and 2-deoxyglucose. In small intestine posttranscriptional regulation of SGLT1 may contribute to glucose-dependent regulation of liver metabolism and intestinal mobility. QCP and QSP are transported by H⁺-peptide cotransporter PepT1 that is colocated with SGLT1 in small intestinal enterocytes. Using coexpression of SGLT1 and PepT1 in Xenopus oocytes or polarized Caco-2 cells that contain both transporters we demonstrated that the tripeptides were effective when applied to the extracellular compartment. After 1 h perfusion of intact rat small intestine with QSP, glucose absorption was reduced by 30%. The data indicate that orally applied tripeptides can be used to down-regulate small intestinal glucose absorption, e.g. in diabetes mellitus.