This paper examines experimentally whether the menstrual cycle of women influences their bargaining behavior and bargaining outcomes. We propose that the menstrual cycle - and possibly the hormones progesterone and oestrogen - influences bargaining behavior. Particularly, we hypothesize that Women bargain more aggressively during the ovulation phase, compared to the other phases, and especially, compared to the premenstrual phase. In our study women are asked to track their menstrual cycle for three months before the experiment. Thereafter, they come to the laboratory experiment to participate in an unstructured bargaining game followed by risk and social preference elicitation tasks. Our results strongly confirm a more aggressive bargaining behavior during the ovulation phase compared to the premenstrual phase. Aggressiveness is measured by the magnitude of the offers and demands made by the players in a bargaining context. Furthermore, we also observed that variations in bargaining behavior over the menstrual cycle don't seem to be explained by variations in risk and social preferences.