Thioridazine hydrochloride, a drug of the phenothiazine group with a piperidine side chain is widely used in human medicine as a psycho-relaxant and anxiolytic agent. The mutagenic effects of thioridazine hydrochloride were studied in Swiss albino mice. The animals were treated with 4, 6 and 8 mg/kg of thioridazine hydrochloride for the micronucleus test, analysis of chromosomes in germ cells, and for sperm morphology assay. Human lymphocyte cultures were treated in vitro with 0.75, 1.25 and 1.75 mg/culture of thioridazine hydrochloride for 24, 48 and 72 hours. The study showed no significant increase in the frequency of micronuclei, chromosomal aberrations in germ cells and sperm head abnormalities in mice, or chromosomal aberrations in human lymphocyte cultures.