Hormones were chemicals secreted by cells in certain parts of the body that triggered changes and responses in other parts or cells of the body. Hormones controlled and regulated activity of cells and organs such as metabolism, reproduction, growth, digestion, and mood. Neurotransmitters were 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")