In vitro, progesterone induces capacitation, hyperactivated motility, and acrosome reactions in sperm, possibly acting through a non-genomic plasma membrane receptor, but neither progesterone functions nor concentration within the mammalian oviduct during the periovulatory period is well characterized. The objectives of this study were to determine the physiological concentrations of progesterone in serum, follicular fluid, and oviductal fluid of golden hamsters during the period of the reproductive cycle in which capacitation and fertilization occur. Fluids were collected from different groups of animals for each of four time points: the normal time for mating (5-6 h post-LH surge [post-LH]), early in capacitation (8-9 h post-LH), immediately preovulation (11-12 h post-LH), and after ovulation near the beginning of fertilization (14-15 h post-LH). Oviductal fluid was collected by 1-h cannulation followed by cardiac puncture for serum collection and follicle aspiration for follicular fluid. Within each time period, the three types of fluid differed significantly in progesterone concentration. Over time, concentrations of progesterone did not change in either serum (range: 5.64-12.85 ng/ml) or follicular fluid (range: 4.2-7.4 micrograms/ml), but the concentration of progesterone in oviductal fluid decreased from 175.06 ng/ml at the first period to 44.01 ng/ml at the fourth (p < 0.05), while the volume of oviductal secretions collected by the same sampling procedure increased from 1.6 to 2.9 microliters (p < 0.05). With this information concerning in vivo concentrations of progesterone during capacitation and fertilization, the physiological role of progesterone in sperm-egg interactions can be addressed.