In W. J. Lynch and M. E. Carroll's (2001) article the authors discuss 3 mechanisms (aversive effects, direct effects, and satiation) that may underlie the regulation of psychoactive drug self-administration. Both P. R. Killeen and M. P. Reilly's (2001) and A. B. Norman and V. L. Tsibulsky's (2001) commentaries are in general agreement with the idea that satiation can be 1 mechanism that regulates drug intake. However, in the latter's opinion, satiation is the only relevant mechanism. G. J. Madden's (2001) commentary highlights the contribution of aversive effects but states that economic costs and benefits may be a better descriptor for the descending limb. J. H. Woods (2001) proposes that because none of these processes are necessary for drugs to function as reinforcers, the study of these mechanisms might not be useful. A. M. Young (2001) suggests that the contribution of any 1 of these mechanisms may be less relevant than interactions between or among them.