Hormones are chemicals secreted by cells in certain parts of the body that trigger changes and responses in other parts or cells of the body. Hormones control and regulate activity of cells and organs such as metabolism, reproduction, growth, digestion, and mood. Neurotransmitters are a type of hormone.
During pon farr, which was a Vulcan time of mating, cortical levels rose and fell, as the brain's regulatory system appeared to shut down when serotonin levels became unbalanced. (VOY: "Blood Fever") In female Vulcans, hormones and endorphins tended to rise to dangerous levels during the pon farr. (ENT: "Bounty")