The serotonergic system, serotonin (5HT), serotonin transporter (SERT), and 
serotonin receptors (5HT-x), is an evolutionarily ancient system that has clear 
physiological advantages to all life forms from bacteria to humans. This review 
focuses on the role of platelet/plasma serotonin and the cardiovascular system 
with minor references to its significant neurotransmitter function. Platelets 
transport and store virtually all plasma serotonin in dense granules. Stored 
serotonin is released from activated platelets and can bind to serotonin 
receptors on platelets and cellular components of the vascular wall to augment 
aggregation and induce vasoconstriction or vasodilation. The vascular 
endothelium is critical to the maintenance of cardiovascular homeostasis. While 
there are numerous ligands, neurological components, and baroreceptors that 
effect vascular tone it is proposed that serotonin and nitric oxide (an 
endothelium relaxing factor) are major players in the regulation of systemic 
blood pressure. Signals not fully defined, to date, that direct serotonin 
binding to one of the 15 identified 5HT receptors versus the transporter, and 
the role platelet/plasma serotonin plays in regulating hypertension within the 
cardiovascular system remain important issues to better understand many diseases 
and to develop new drugs. Also, expanded research of these pathways in lower 
life-forms may serve as important model systems to further our understanding of 
the evolution and mechanisms of action of serotonin.