Genetic toxicology of lysergic acid diethylamide (LSD-25)." The acute and the chronic psychotomimetic potentials of the hallucinogen lysergic acid diethylamide (LSD-25) have been recognized for almost 40 years. That additional types of the biological effects should have come under scrutiny was directly attributable to widespread use and abuse of this drug on a world-wide basis. Although ""genetic toxicology"" encompasses a broad spectrum of disciplines, including many areas of highly specialized research, perhaps the most germane, and those on which this review has concentrated, are Clastogenicity, Mutagenicity, Teratogenicity and Oncogenicity. Based on our current understanding and interpretation of the available data, the genetic toxicology of LSD provides an excellent example of Newton''s ""third law of motion"", e.g., to every force there is an equal and opposite reaction force. From the published material it is impossible to draw clear cut conclusions regarding any of the above ""problem areas"" in spite of the considerable scientific effort invested. Most of the in vitro studies performed on the clastogenicity of LSD indicate either suppression of mitosis or enhanced chromosome damage. However, extrapolation of such results to the in vivo situation is very difficult. With regard to in vivo human use of the drug, no concensus is attainable as to chromosome breakage and the inconsistencies within and between studies remain inexplicable. However, several of the ""controlled"" investigations assessing the in vivo effect of chemically pure LSD suggest a transient increase in lymphocyte chromosome breakage. On the other hand, the results of cytogenetic studies on experimental animals are contradictory. Although human studies are nonexistent, in those experimental organisms tested, using accepted techniques, LSD proved to be, at best, a weak mutagen, if mutagenic at all. Teratogenicity studies in animals are confusing due to the multitude of organisms and plethora of discriminant parameters studied. However, with regard to man there has been ample opportunity and one can conclude that LSD is not teratogenic. As to the drug''s oncogenic potential, the 3 reported cases of leukemia in LSD users are most likely the result of coincidence."