Several investigators have described the altered sleep patterns in Down's syndrome subjects. The most relevant findings have been a reduction in percentage of REM sleep, a prolonged latency to the first REM episode, an increase in undifferentiated sleep, and a reduced ratio of the oculomotor frequencies. Because it is of interest to identify new drugs able to increase the percentage of REM sleep in mentally retarded subjects, we studied the effects of butoctamide hydrogen succinate (BAHS) on nocturnal sleep in eight young institutionalized Down's syndrome subjects. BAHS produced a significant increase in the percentage of REM sleep as well as a decrease in undifferentiated sleep and latency to the first REM.