Depression is a common problem in elderly patients and frequently is treated with antidepressants. We present the case of a 67-year-old depressed woman who began treatment with citalopram. Two months later, hyponatremia was diagnosed, most likely syndrome of inappropriate antidiuretic hormone secretion. After discontinuation of citalopram therapy, serum sodium concentrations normalized. Later, she began treatment with mirtazapine. Five months after initiating mirtazapine therapy, she developed symptomatic hyponatremia. After mirtazapine therapy was discontinued, serum sodium concentrations normalized. In this case, unlike those previously reported, hyponatremia recurred 5 months after switching from citalopram to mirtazapine, which is believed to be a safe antidepressant. In conclusion, patients older than 60 years should have baseline electrolyte measurements before starting therapy with an antidepressant, and these should be monitored not only in the first weeks of treatment, but throughout the full course.