To the Editor: The article by Jay D. Amsterdam, M.D., and his colleagues R5715511CHDBJDEH on fluoxetine plasma levels was impressive and interesting. The authors, however, state that "it is possible . . . that a lower dose would have shown a threshold effect" (p. 967). I think it would be more accurate to say, on the basis of elementary pharmacology, that a lower dose would have to show a threshold effect if the medication is, in fact, effective; an ineffective dose might be 1 mg or 1 molecule, but there has to be a dose that is too small to be effective. This is not an empirical issue, and one cannot question the fact that an ineffective dose would have been found had the authors used lower doses.