During a recent investigation of the effects of lysergic acid diethylamide (LSD) on human chromosomes, three individuals, included as controls, showed elevated frequencies of chromosomal breakage while being treated with chlorpromazine.1 Subsequently, invitro observations on human leukocytes and fibroblasts suggested that diazepam is also capable of inducing chromosome damage.2 Due to the widespread clinical use of such drugs in our population, an in-vivo and invitro cytogenetic study of several tranquilizing agents was carried out. Materials and Methods: In Vitro Studies.—Peripheral leukocyte cultures were initiated from two normal, healthy donors (one male and one female) with no known history of prior drug ingestion, radiation exposure, or recent viral infection. Chromosomes were obtained from cells of whole-blood inoculum (3 to 4 drops), grown in commercially available microculture kits, and incubated at 37 C for 72 hours. Two hours before harvest, colcemide was added at a final concentration of 0.05μg/ml