threshold is the odorant do not produce in concentration above which further increases creases in perceived intensity. The detection or absolute threshold is the minimum odorant concentration that can be distinguished from an environment free of that odor. Correspond- ingly, the recognition threshold is the minimum concentration at which an odorant can be individually identified. The recognition threshold of a particular odorant is never lower than its absolute threshold. Thresholds are not firmly fixed values. Sensitivity fluctuates irregularly, and a certain odorant concentration will elicit a response at one time but not at another. Classical psy- chophysicists recognized the instability inherent in sensitivity, and they defined the abso- lute threshold statistically as that odorant concentration that is detected as often as not over a series of presentations. The probability of detection of such a stimulus is 50%. This definition of threshold has prevailed in the literature of air pollution and industrial hygiene. The theory of signal detection, originally engineered for telephone and radio communi cation systems by Shannon and Weaver(4) and later translated into a more general theory by Swets and others(5), contributed significantly to the development of modern psycho- physics by specifying the dependence of threshold determinations on certain experimental variables. Detection theory has revealed the criteria employed by subjects in making per- ceptual judgments.