. Subcutaneous injections of opiates produced the Straub tail reaction in mice. The potencies of the opiates in mice were consistent with previous estimates of the analgesic potencies in animals and in man. . The potencies of sixteen antagonists in counteracting the reaction were consistent with those previously obtained with the rat tail‐flick test. . The (–) isomers of four benzomorphan derivatives were much more potent in counteracting the reaction than their (+) isomers and about twice as potent as their racemates. The activity of the isomers seemed to follow Pfeiffer's rule: the lower the effective dose of a drug, the greater the difference in the pharmacological effects of the optical isomers. One of the trans isomers acted like an opiate, while its cis isomer acted like an antagonist. . Naloxone and nalorphine fulfilled conventional criteria for competitive antagonism, whereas atropine and the (–) and the (+) isomers of pentazocine and of cyclazocine did not do so. . The Straub tail test seems to be useful for studying structure‐activity relations among opiates and opiate antagonists.

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