P-Glycoprotein (P-gp) is an efflux transporter which is up-regulated at the blood–brain barrier in both morphine-and oxycodone-tolerant rats. Numerous studies have shown that many clinically employed opioid analgesics are substrates for P-gp, suggesting that up- regulation of P-gp may contribute to the development of central tolerance to opioids. The studies herein focus on the development of SAR for P-gp substrate activity in the ...