SB408124 is a potent and selective small molecule antagonist of the orexin-1 (OX1) receptor, exhibiting Ki values of 57 nM and 27 nM in whole cell and membrane assays, respectively, with 50-fold selectivity over the OX2 receptor. This compound is a valuable research tool for investigating orexin system function in sleep, addiction, and stress-related behaviors in both in vitro and in vivo models.