According to the literature, various occupational and environmental stressors may cause alterations in serotonin (5-HT) turnover and in its principal metabolite, 5-hydroxy-3-indoleacetic acid (5-HIAA). The aim of this study is to evaluate whether traffic police exposed to urban pollutants and possible psycho-social stressors could be at risk of alterations in urinary 5-HIAA in 24 hours (5-HIAA[U]) compared with a control group. After the main non-occupational confounding factors were excluded, 5-HIAA(U) excretion was investigated in 140 employees of a municipal police force: 70 traffic police with outdoor activity that exposed them to urban pollutants and 70 administrative workers with indoor activity. Subjects were matched by sex, age, and length of working life. The mean 5-HIAA(U) levels were significantly lower in traffic police than in administrative workers, both males and females (p = .025, and p = .027, respectively), matching modifications in 5-HIAA(U) levels found by other authors in studies on animals and human subjects.