Carnitine/organic cation transporter OCTN1 (SLC22A4) is expressed in ubiquitous organs. OCTN1 recognizes various therapeutic agents as substrates in vitro. However, the role of OCTN1 in absorption, distribution and excretion of nutrients or therapeutic agents in vivo had been minimally understood. Recently, we generated octn1 gene knockout (octn1^<-/->) mice to identify the physiological and/or pharmacological roles of OCTN1 in vivo. We then clarified a naturally occurring antioxidant ergothioneine as one of the in vivo substrates of OCTN1. In this review, we focused on (1) fundamental role of OCTN1 in tissue distribution and renal tubular reabsorpion of ergothioneine, (2) possible relationship between OCTN1 and human diseases (Crohn's disease and rheumatism), (3) microscopic localization of OCTN1 inside the tissues, (4) bidirectional function of OCTNs (influx of nutrients and efflux of xenobiotics), and (5) protein-protein interaction of OCTN1 with PDZ adaptor proteins.