Two concepts are especially important in understanding the action of agonist‐antagonist opioid analgesics. First, knowing to which receptor classes an agonist‐antagonist binds permits prediction of its pharmacodynamics. For instance, psychotomimetic effects occur with binding to specific opiate (k) and nonopiate (Σ) receptors. Second, to have limited agonist activity despite increasing dose ceiling effect, the antagonist effect usually is of the same receptor class as the agonist effect. 1991 Pharmacotherapy Publications Inc.