In this commentary the authors argue that the satiety threshold is the only mechanism that is sufficient and necessary to explain the regulation of maintained self-administration. The other mechanisms have been proposed mainly because of 2 sources of confusion surrounding the self-administration paradigm: the failure to distinguish between separate phases of a self-administration session and the assumption that positive reinforcement underlies drug self-administration. The authors of this commentary emphasize that the direct effects and aversion mechanisms have been proven to be untenable for the reasons reviewed by W. J. Lynch and M. E. Carroll (2001) and that the "ascending limb" of the dose-response curve is an experimental artifact. These ideas have persisted only to salvage a role for positive reinforcement in drug self-administration. The authors conclude that reinforcement is not relevant to the regulation of maintained self-administration.