Presence, uptake and production of serotonin and its effect on the production of other hormones were studied using immunocytochemical flow cytometric method. In a serotonin (10–12 M) containing medium up to 15 min. serotonin level does not elevate in the cells, but after 30 min. there is a significant elevation which remains till 4 h. In cells starved in salt solution the elevation is higher which calls attention to the effect of (starvation) stress. Using four enzyme blockers tryptophane hydroxylase inhibitor PCPA decreased (in serotonincontaining medium) and MAO B blocker deprenyl increased serotonin content, while serotonin reuptake inhibitor fluoxetine and MAO-A blocker clorgyline were ineffective. Extremely low concentrations of serotonin (10–15 M in case of histamine and 10–18 M in case of ACTH and T3) in the milieu was sufficient for increasing hormone (ACTH, T3, histamine) levels inside the cells. In conclusion; serotonin can be taken up by the cells and can be produced by induction, as Tetrahymena has enzymes for building it up and decomposing it. For synthesizing serotonin; basic molecules from outside are not needed. Serotonin in a minute amount can induce production of different hormones.