Using a conditioning paradigm, the olfactory sensitivity of CD-1 mice for a homologous series of aliphatic aldehydes (n-butanal to n-nonanal) was investigated. With all six odorants, the animals significantly discriminated concentrations below 4 ppb (parts per billion) from the odorless solvent, and with n-butanal, n-heptanal, and n-nonanal the best-scoring animals were even able to detect concentrations below 40 ppt (parts per trillion). The results showed (a) CD-1 mice to have a well-developed olfactory sensitivity for aliphatic aldehydes which for the majority of substances is higher than that of other mammalian species, (b) a limited concentration range of about 1 log unit from threshold to near saturation which is similar to that shown at the single receptor cell level, (c) no significant correlation between perceptibility in terms of olfactory detection thresholds and carbon chain length of the aldehydes tested, and (d) no systematic effect of preexposure to an aldehyde on sensitivity. These findings suggest that CD-1 mice are highly sensitive to non-pheromonal odorants that are abundant in plant odors. Further, the results demonstrate that sensitivity for members of a homologous series of substances is not a simple function of vapor pressure as threshold values did not correlate with molecular weight. These threshold data may provide useful information for the choice of adequate stimulus concentrations in electrophysiological or imaging studies of the olfactory system or investigations of the discriminative abilities of the mouse.