An impairment of the monoaminergic systems has frequently been reported for Alzheimer's disease (AD) as well as an overactivity of cerebral monoamineoxidase B (MAO-B). L-deprenyl (LD), a selective and irreversible MAO-B inhibitor, has recently been proposed for the treatment of AD. The cerebrospinal fluid (CSF) levels of norepinephrine (NE), epinephrine (E), dopamine (DA), 3-methoxy-4-hydroxyphenylglycol (MHPG), 5-hydroxyindoleacetic acid (5-HIAA), and homovanillic acid (HVA) were studied in 14 patients suffering from dementia of Alzheimer type (DAT) and in 14 controls. A three-month double-blind study comparing LD with placebo was carried out, in the DAT group, and the influence of the treatment on neurotransmitter levels and cognitive performance was evaluated. The basal study revealed a significant reduction in CSF NE and HVA levels in DAT patients when compared with controls; the treatment with LD determined a significant decrease in HVA levels only and, as to neuropsychological investigation, a global amelioration of cognitive performances.