The serotonin system is a set of neural pathways located in the brainstem and hypothalamus, predominantly fuelled by the neurotransmitter serotonin, which is a key messenger in the regulation of mood, emotion, stress response, and many other cognitive and behavioural functions. This system is pivotal in the functioning of the central nervous system, with evidence that suggests dysfunction of the serotonin system is linked to a range of diverse mental health issues, such as depression, anxiety, bipolar disorder, and attention-deficit/hyperactivity disorder.

See also: brain stem, peak experience, psychedelic experience, altered state