A double-blind, placebo-controlled study assessed subjective effects of smoking marijuana with either a long or short breath-holding duration. During eight test sessions, 55 male volunteers made repeated ratings of subjective "high," sedation, and stimulation, as well as rating their perceptions of motivation and performance on cognitive tests. The major finding of the study was that the long, relative to the short, breath-holding duration increased "high" ratings after smoking marijuana, but not placebo. Marijuana smoking increased sedation and a perception of worsened test performance, and decreased motivation with respect to test performance. Paradoxical subjective effects were observed in that subjects reported some stimulation as well as sedation after smoking marijuana, particularly with the long breath-holding duration. Breath-holding duration did not produce any subjective effects that were independent of the drug treatment, i.e.. occurred equally after smoking of marijuana and placebo, such as we previously observed with respect to test performance.