Norbuprenorphine is a major metabolite of buprenorphine and potent agonist of μ, δ, and κ opioid receptors. Norbuprenorphine is the only known active metabolite of buprenorphine. It has been shown to be a respiratory depressant in rats, but only at concentrations at least 50-fold greater than those observed following application to humans of BuTrans 20 mcg/h. Compared with buprenorphine, norbuprenorphine causes minimal antinociception but greater respiratory depression. In rats, nor-buprenorphine had 1/50th the analgesic potency of buprenor-phine but 10-fold greater respiratory depressant potency. Norbuprenorphine is an in vitro substrate of the efflux transporter P-glycoprotein (Mdr1). Norbuprenorphine is an in vitro and in vivo substrate of P-glycoprotein. P-glycoprotein-mediated efflux influences brain access and antinociceptive, but not the respiratory, effects of norbuprenorphine.