The present study was designed to shed light on a) the cytogenetic effects of metronidazole and aquatic extract of Pelargonium odoratissimum (L.) soland on laboratory mice (in vivo)and in human blood lymphocytes (in vitro) b)chemical analysis of P. odoratissimum for different active compounds . It was also aimed to investigate the role of P. odoratissimum aquatic extracts in reducing the cytogenetic effects of metronidazole in mice and human blood lymphocytes. The examinations that conducted in mice, were mitotic index, chromosomal aberrations in bone marrow and study the total and differential count of leukocytes, while human blood lymphocyte culture studies (in vitro) were mitotic index , balsto index, replecative index, sister chromatid exchange and chromosomal aberrations. The cytogenetic effects of the drug and plant aquatic extracts were investigated after four days of treatment in mice with four different doses from each. For metronidazole, they were (1, 100, 200 or 400mg/kg ), while doses of P. odoratissimum used were(100, 200 , 400 or 800mg/kg ). The concentrations for mteronidazole for human blood lymphocyte culture were (2.5 , 5, 10, 20, 40 or 80µg/ml), and for P. odoratissimum aqueous extract were ( 5, 10, 20, 40 80 or 100µg/ml). An interaction between plant extract and metronidazole (high concentration) was carried out through three types of treatments (before, after and mixture of plant extract and drug treatment) to determine the activity of P. odoratissimum aqueous extract in preventing or reducing the drug side effects both in vitro and in vivo. The following results were obtained: 1.Chemical investigation showed that P. odoratissimum contains two different classes of active compounds namely, flavonoids and saponins. 2.Metronidazole revealed clear effects, in reducing mitotic activity and increased spontaneous chromosomal aberrations in mouse bone marrow cells (in vivo) and in human blood lymphocytes (in vitro) causing a reduction in mitotic index, balsto index and replecative index and induction of sister chromatid exchange and chromosomal aberrations. These effects were proportional with the concentrations; a phenomenon, which suggested that drug, has a genotoxic effect. 3. Aquatic extract of P. odoratissimum had a significant genotoxic effects at high doses on the mouse bone marrow cells (in vivo) and human blood lymphocytes culture (in vitro). 4. Aquatic extract of P. odoratissimum at low dose ,however, showed a protective value against the genotoxic effect of metronidazole. In moue bone marrow cells and human blood lymphocyte culture, this was more pronounced in pre-treatment and simultaneous treatment than in post-treatment. So P. odoratissimum aquatic extract is consider as desmutagen in the first order and bioantimutagen in the second order, as a result for its ability to repair CA and increase MI in mouse system and in human blood lymphocyte culture system . It also had the ability to increase BI and RI and decease SCE in human blood lymphocytes culture (in vitro). 5. P. odoratissimum aquatic extract showed no effect on mouse blood leukocyte differential count. 6. Metronidazole also showed no effect on mouse blood leukocyte differential count.