We investigated the antidepressant efficacy of l-deprenyl (selegiline), a selective monoamine oxidase B inhibitor (MAOI), in a six-week open trial of 17 patients with atypical depression. Such patients have previously been shown to benefit from nonselective MAOIs such as phenelzine sulfate. Ten patients (59%) responded to l-deprenyl, but nine required dosages above the 10 to 20 mg/day used in previous investigations. l-Deprenyl was superior to six weeks of placebo administered to diagnostically similar patients in a separate double-blind study. In contrast with previous findings with pheneizine, responders to l-deprenyl differed from nonresponders by having lower baseline anxiety ratings. Even at high dosages, there appeared to be fewer side effects with l-deprenyl than with nonselective MAOIs.