A novel brain-selective acetylcholinesterase inhibitor, SDZ ENA 713, is under development for the treatment of dementia of the Alzheimer type. To determine the threshold dose for central activity, single doses of the compound were administered to 20 young male volunteers in a double-blind cross-over design and the effects on the sleep electroencephalography studied. The first group of eight volunteers received in random order: placebo, 0.5 mg; and 1 mg SDZ ENA 713. The second group of 12 volunteers received: placebo, 1.3 mg; and 2 mg SDZ ENA 713. Sleep quality was not affected by the study medication, which was well tolerated by all subjects. A statistically significant increase in rapid-eye movement sleep density was observed after doses of 1 mg, 1.3 mg, and 2 mg. Rapid-eye movement latency and slow-wave sleep were not altered. The results demonstrate that SDZ ENA 713 is centrally active in man at well-tolerated doses.