Monoamine neurotransmitter systems, along with cholinergic systems, are known to play important roles in cognition, and are disrupted in at least some patients with dementia of the Alzheimer type (DAT). This suggests that monoamine-enhancing drugs might ameliorate cognitive symptoms in certain patients with DAT. L-Deprenyl is a monoamine oxidase (MAO) inhibitor which may selectively inhibit MAO-B at low doses, while at high doses it nonselectively inhibits MAO-A as well as MAO-B. We studied its effects on several types of cognitive function in 17 patients with DAT. Two doses of L-deprenyl (10 mg/day and 40 mg/day) and placebo were compared in a double-blind, serial treatment design. Episodic learning and memory, knowledge memory, attention, recognition, and performance on a continuous performance task were assessed at baseline and under these drug and placebo conditions. Statistically significant improvement was noted in performance on an episodic memory and learning task requiring complex information processing and sustained conscious effort during treatment with L-deprenyl 10 mg/day. Knowledge memory, intrusions, and other cognitive functions relevant to DAT were not altered by L-deprenyl at either dose.