Pretreatment urinary MHPG levels were examined in 28 depressed patients as a possible predictor of response to treatment with maprotiline, a tetracyclic antidepressant that exerts potent effects on norepinephrine uptake, but has little effect on serotonin uptake. Maprotiline was administered in doses up to 150 mg/day during the first 2 weeks after which time the dose could be increased incrementally up to 300 mg/day if indicated clinically. At 2 weeks, patients with low pretreatment urinary MHPG levels responded more favorably to treatment than did patients with high MHPG levels. At 4 weeks, patients with low MHPG levels continued to show more favorable responses; however, differences between the two groups were less clear-cut than at 2 weeks. The findings suggest that patients with low pretreatment urinary MHPG levels are more sensitive to, and respond more rapidly to, treatment with maprotiline than patients with high pretreatment urinary MHPG levels.