Raised brain levels of testosterone (Tes), as well as single nucleotide polymorphisms of P-glycoprotein (P-gp) that cause impaired transport function, are associated with increased risk of suicide. Here, we examined whether Tes and its precursors and metabolites are substrates of P-gp, using several in vitro methods. In ATPase assay, increased ATP consumption was observed as the concentrations of Tes, dihydroepiandrosterone (Dhea), androstenedione (Ado), and dihydrotestosterone (Dht), but not androstenediol (Adol), were increased, suggesting that these four androgens are transported by P-gp. Furthermore, Tes and Ado, though not Dhea or Dht, increased the intracellular accumulation of Rhodamine 123 (Rho123), a typical substrate of P-gp, in a P-gp-overexpressing cell line, suggesting that they inhibit Rho123 efflux and thus are substrates or inhibitors of P-gp. A membrane permeability study using P-gp-overexpressing cells in Transwell inserts indicated that the permeability coefficients of both Ado and Tes in the basal-to-apical direction (excretion) are significantly higher than those in the apical-to-basal direction. Moreover, transport of both Ado and Tes was significantly suppressed by verapamil, a typical P-gp inhibitor. These results indicate that Tes and Ado are endogenous substrates of P-gp. These findings provide a physiological basis for understanding previously reported associations of P-gp dysfunction and raised brain levels of Tes with suicidal behavior, and may open up new possibilities for treating patients at risk of suicide. (C) 2019 Elsevier Inc. All rights reserved.