Serotonin is primarily produced in the enterochromaffin cells of the gastrointestinal tract, specifically in the small intestine.

While serotonin is often associated with the brain and mood, it is actually produced in much greater quantities in the gut. It plays a crucial role in regulating digestion, appetite, and intestinal motility.

However, it's important to note that the brain also produces serotonin, but in smaller amounts compared to the gut. This brain-derived serotonin is involved in mood regulation, sleep, appetite, and other cognitive functions.