The levels of 3-methoxy-4-hydroxyphenylethylene-glycol (MHPG) in cerebrospinal fluid (CSF), before and after treatment with imipramine, were determined in 12 patients with endogenous depression. No significant change as observed in the whole group of patients, but patients with higher CSF-MHPG basal values showed better therapeutic results and a significant decrease of the norepinephrine catabolyte. The possible relevance of norepinephrine system imbalance to the therapeutic response to imipramine is discussed.