Serotonin is a neurotransmitter that important in maintaining the integrity of body functions, regulating mood, emotion, sleep, appetite, and regulating memory function. However, changes in serotonin and its receptors are associated with the development of anxiety, depression, and psychiatric disorders, such as schizophrenia. Exercise is suggested as a treatment for several psychiatric disorders. The study aims to prove the effect of moderate-intensity swimming exercise for 4 weeks on increasing serotonin levels in male mice (Mus musculus). The study was a true experiment with research design from a randomized control group of posttest-only designs using 20 male mice (Mus musculus), eight weeks of age, weighing 30-35 grams and randomly divided into two groups, that is CTL (n = 10, control group), MIE (n = 10, moderate-intensity swimming exercise). Moderate-intensity swimming exercise is carried out with a frequency of 3 times/a weeks for 4 weeks. Measurement of serotonin levels using the Enzyme-Linked Immunosorbent Assay (ELISA) method. Data is analyzed using the Independent Samples T-Test with a significant level (p ? 0.01). The results showed a significant difference in serotonin levels between MIE and CTL (100.61?20.08 vs. 57.06?5.13 ng/mL (p ? 0.001)). Based on the results of the study concluded that moderate-intensity swimming exercise increases serotonin levels