Investigating the effects of marijuana on human psychological functioning, this study differs from previous research in two ways: 1) it is concerned with relatively complex cognitive processes; 2) it has a theoretical rationale. The general hypothesis of the study states that marijuana will impair its user's ability to form and use abstract concepts. Twelve adult, male, experienced volunteer users of marijuana served as subjects for the study. To require the subjects to form and use abstract concepts in a variety of ways, the study administered the following tests: letter series, word grouping, conceptual clustering memory, closure speed, embedded figures, size-weight illusion, water-jar, hidden word, and anagram. Of the seven tests, results partially or strongly supported five, failed to support one, and rejected one. In general, the study demonstrated drug-induced impairments with dosage level constituting an important factor in determining the results. (Author/LAA)