We report a patient who developed probable serotonin syndrome after taking amitriptyline and paroxetine. A 78-year-old man suffered from post-herpetic neuralgia since 3 years before. He had been treated with repeated stellate ganglion blocks and amitriptyline 30mg/day. He subsequently developed disorientation and tremor 14 days after taking amitriptyline. The tremor spontaneously resolved 3 days after cessation of amitriptyline. Paroxetine 20mg/day was begun 7 days after cessation of amitriptyline. Upper respiratory symptoms occurred 3 days before this medication. Thereafter, he developed tremors of the upper limbs and torso, myoclonus, and exaggerated deep tendon reflexes. These symptoms disappeared 6 days after withdrawal of paroxetine. There was no abnormal finding in the cerebrospinal fluid. Clinical symptoms and signs strongly suggested that the patient developed serotonin syndrome. Physicians should be alert that amitriptyline and paroxetine may cause serotonin syndrome.