Oral administration dominates contemporary drug therapy and will most likely continue to do so as it is considered to be safe, efficient and easily accessible with minimal discomfort to the patient compared to other routes of administration such as intramuscular, subcutaneous, rectal and pulmonary delivery. However, despite these advantages, many of the mechanisms of drug uptake following oral administration remain to be fully characterized. In drug discovery and preclinical development there is a strong demand for the accurate and rapid characterization of processes such as absorption, distribution, metabolism and excretion. These biopharmaceutical/pharmacokinetic variables should also be related to pharmacodynamic and toxicological variables such potency and duration of effect. Although these processes are highly dynamic and complex, they are not yet fully characterized in vivo. Various in vitro pharmacokinetic screening methodologies have significantly increased the amount of experimental data generated in this part of the drug discovery process. In addition to these techniques, there is a strong need for in silico methods that may be used to accurately predict pharmacokinetic properties from molecular structure. For instance, pharmacokinetic filters that can sort out compounds with undesirable pharmacokinetic properties can be applied to virtual screening or compound design to reduce attrition rates. The aim of this review is to summarize reported human permeability values and to evaluate how they correlate to corresponding rat intestinal permeability data obtained in single-pass perfusion and Ussing Chamber experiments. The human permeability data are based on direct in vivo determinations in the human gastrointestinal tract with a single-pass perfusion system. The focus of this attention is particularly justified as the availability of directly determined in vivo permeability data in the literature is limited. In addition, there is a shortage of intestinal permeability studies in other mammals.