PEPT1 is a high-capacity, low-affinity peptide transporter that mediates the uptake of di- and tripeptides in the intestine and kidney. PEPT1 also has significance in its ability to transport therapeutic agents and because of its potential as a target for anti-inflammatory therapies. To further understand the relevance of specific peptide transporters in intestinal physiology, pharmacology and pathophysiology, we have generated Pept1 null mice by targeted gene disruption. The Pept1 gene was disrupted by insertion of a lacZ reporter gene under the control of the endogenous Pept1 promoter. Phenotypic profiling of wild-type and Pept1 null mice was then performed, along with in vitro intestinal uptake, in situ intestinal perfusion and in vivo pharmacokinetic studies of glycylsarcosine (GlySar). Pept1 null mice lacked expression of PEPT1 protein in the intestine and kidney, tissues in which this peptide transporter is normally expressed. Pept1-deficient mice were found to be viable, fertile, grew to normal size and weight, and were without any obvious abnormalities. Nevertheless, Pept1 deletion dramatically reduced the intestinal uptake and effective permeability of the model dipeptide GlySar (i.e., by at least 80%), and its oral absorption following gastric gavage (i.e., by about 50%). In contrast, the plasma profiles of GlySar were almost superimposable between wild-type and Pept1 null animals after intravenous dosing. These novel findings provide strong evidence that PEPT1 has a major role in the in vivo oral absorption of dipeptides.