The neuroleptic threshold hypothesis states that the first appearance of slight increases in muscle tone during the initiation of neuroleptic treatment heralds the achievement of neuroleptic levels in the brain which are biologically significant and capable of provoking the poorly understood changes leading to therapeutic response. The hypothesis has not been adequately tested. If shown to be true, it represents a potential basis for rational prescribing of neuroleptic drugs. It is, at least, an empirical, testable question for the field of psychopharmacology to finally resolve. © 1986.