Mammalian urine releases complex mixtures of volatile compounds that are used in reproduction, territoriality and conspecific recognition. To understand how such complex mixtures are represented in the main olfactory bulb, we analysed the electrophysiological responses of individual mitral cells to volatile compounds in mouse urine. In both males and females, urine volatile compounds evoke robust responses in a small subset of mitral cells. Fractionation of the volatile compounds using gas chromatography showed that out of the hundreds of compounds present, mitral cells are activated by single compounds. One cohort of mitral cells responded exclusively to male urine; these neurons were activated by (methylthio)methanethiol, a potent, previously unknown semiochemical present only in male urine. When added to urine, synthetic (methylthio)methanethiol significantly enhances urine attractiveness to female mice. We conclude that mitral cells represent natural odorant stimuli by acting as selective feature detectors, and that their activation is largely independent of the presence of other components in the olfactory stimulus.