The unicellular Tetrahymena pyriformis was stressed by 37°C heat for 1 h and its hormone (serotonin, histamine, triiodothyronine) content was measured by immunocytochemical flow cytometry in different time points (immediately after treatment and after 1, 2, 8, 16 weeks). The treatment increased each hormone level for two weeks, however, after 8 weeks the hormone concentration inside the cells decreased and in case of serotonin this was similar in the 16th week, while the other two hormones' level was similar to the control. Insulin further increased the hormone production during treatment, but this effect was not durable. After one week the cells behave similar to those, subjected to heath shock only. The results show that a single stress causes deep and durable changes in the hormone household of Tetrahymena which is influenced by exogenously given insulin only in the acute phase.