The spermatozoa delivered to the female genital tract need to swim towards the oocyte through viscous secretions. Once close to the oocyte, the spermatozoa are guided by a gradient of progesterone (P4) and other unknown chemoattractants via a process known as chemotaxis. Using polyvinylpyrrolidone to establish the conditions of viscosity, we examined the response of mouse spermatozoa to P4 Herein, we show that in low-viscous media, P4 induces hyperactive-like motility whereby sperm show erratic trajectories and non-progressive movement. However, an opposite response is produced in viscous medium in that trajectories are linear and motility is more progressive and less erratic. Our observations provide a behavioural explanation for the chemotaxis of spermatozoa swimming under viscous conditions in a spatial gradient of the chemoattractant P4 They also highlight the importance of using viscous solutions to mimic in vivo conditions when analysing sperm behaviour in response to any stimulus.