This study was conducted with the aim of determining the effects of periodical hormonal fluctuations related to the menstrual cycle on chromosome sensitivity and cytotoxicity. The study group consisted of 8 healthy donors (4 nonsmokers and 4 smokers). Cytogenetic tests were done in vitro (in test tubes), and the known mutagenic effect of mitomycin C was added to determine chromosome sensitivity. Blood was drawn from the donors at specified time intervals (follicular phase, ovulation phase, and luteal phase), and control groups and mitomycin C-treated groups were formed. In the controls, the highest sister chromatid exchange frequency was detected in the follicular phase and the lowest frequency was detected in the luteal phase of nonsmokers. In smokers, the highest sister chromatid exchange frequency was detected in the ovulation phase and the lowest frequency was detected in the luteal phase. In terms of chromosomal aberrations, the highest values were detected in the follicular phase and the lowest values were detected in the luteal phase in nonsmokers. On the contrary, the highest rate of anomaly was detected in the luteal phase and the lowest rate of anomaly was detected in the follicular phase in smokers. However, there was no statistically significant difference between these findings. The data of the MMC application were similar in both groups. In this study, both the follicular phase and the ovulation phase showed slightly higher chromosome sensitivity, while the chromosomes in the luteal phase were the most stable. These results are probably due to hormonal fluctuation.