Flehmen is the term used to describe the behavior in which a horse extends its neck, raises its head, and inhales as it rolls its upper lip back, displaying its front teeth.

Sharon Crowell-Davis, DVM, PhD, Dipl. ACVB explains that horses display the flehmen response to facilitate transfer of inhaled scent molecules (pheromones and possibly some other substances) into the vomeronasal organ (VNO), a specialized chemosensory structure found in many mammals.

Once known as Jacobson's organ, the VNO has different physical forms and locations in various species of animals. In general, it is connected with the mouth, nasal passages, or both. When certain scents impact the VNO, signals are transmitted to centers in the brain's accessory olfactory bulbs. Depending on the scent detected, various physiologic and behavioral reactions may then be triggered.

Stallions show the flehmen response most frequently, often after sniffing a mare's urine or simply when they are in the presence of a mare in estrus. Visual cues may be important in the stallion's response, as stallions whose vision was blocked showed a lower frequency of flehmen than stallions that could watch mares urinating. "It appears that flehmen facilitates chemosensory priming of stallions for reproductive behavior, rather than being an immediate component of sexual behavior," said Crowell-Davis