The present invention relates to the fields of molecular medicine and targeted delivery of therapeutic or diagnostic agents to cells outside the vascular system and into the parenchymal tissue of organs within the body. More specifically, the present invention relates to the methods used to identify membrane receptors or transporters capable of carrying cargo specifically targeted to the parenchymal tissue of the brain and to in vivo enrichment methods for selecting peptides that are transported across the blood-brain barrier (“BBB”), or analogously, across other membrane containing organs or structures, such as liver, spleen, kidney and tumors.