IT is very difficult to define the mechanism by which olfactory receptors on male moth antennae perceive a sex attractant chemical because of the various unsubstantiated olfaction theories, and also because of conflicting reports of the ability of chemicals, structurally different from natural sex attractants, to either inhibit1–5 or mimic6–9 attractant activity. Using the synthetic red-banded leaf roller moth (Argyrotaenia velutinana) sex attractant (RiBLuRe)10, we have been able to obtain evidence that the geometrical isomer inhibits the response of males to the sex attractant chemical, and that varying degrees of inhibition can be elicited with positional isomers, homologues and analogues. The preliminary data support a mechanism of perception in which the sex attractant, a chemical with the correct spatial arrangement of active sites, possesses the affinity and intrinsic activity for the receptor sites to elicit proper behavioural responses, including flight to the location of the sex attractant. Closely related isomers which do not display intrinsic activity by attracting males apparently can possess a strong affinity for the receptor sites, thus acting as inhibitors or modifiers of the sensory input.