AbstractOpioid receptors mediate the pharmacologic actions of morphine‐like analgesics, a class of drugs that are important clinical agents which are also widely abused socially as a result of their euphorogenic and addictive properties. Cloning of the opioid receptors has confirmed the original postulate that there are only three classes of opioid receptors: µ, δ, and κ. The most commonly used isolated tissue preparations for measuring activity at these receptors are the guinea pig ileum and the mouse vas deferens. This unit describes preparation of these tissues to measure agonist and antagonist properties of test compounds at the µ‐ and κ‐opioid receptors in guinea pig ileum and the d‐opioid receptor in mouse vas deferens.