This study describes the effect of two weeks of delta-9-tetrahydrocannabinol (THC) administration upon normal sleep. The two subjects, two brothers in their 20s, slept in the laboratory for 27 consecutive nights and then, after four nights at home, for four additional nights. One subject, after an adaption night, received placebo for four baseline nights, 30 mg of THC for the next 14 nights, and placebo during four withdrawal nights. The other subject received placebo during this entire period. One year later the subjects alternated these conditions. The subjects had difficulty falling and staying asleep during the first two nights of placebo after 14 consecutive drug nights. This mild drug withdrawal insomnia was not accompanied by the increase of REM sleep which frequently accompanies withdrawal of other drugs. Starting after about a week of THC administration, and continuing for a week after drug discontinuance, there was a marked decrease in the type of sleep associated with slow waves in the electroencephalogram, nonREM sleep stages 3 and 4. The fact that prolonged, but not acute use, suppresses slow wave sleep indicates that this commonly used drug produces a poorly understood change in brain physiology.