Ciliary beat frequency (CBF) was measured in slice preparations of the Fallopian tube fimbria, using videomicroscopy with a high-speed (500 Hz) camera in guinea pigs that were treated with β-oestradiol benzoate (βE2B) and medroxy progesterone (mPRG). In non-ovulating guinea pigs at 4 weeks of age, the CBF of the fimbria was high (17.8 Hz). In sexually mature guinea pigs (12-16 weeks of age) with constant ovulation, the CBF varied from 12 Hz to 16 Hz. The in vivo administration of both ICI-182,780 (a blocker of βE2 receptors) and mifepristone (a blocker of PRG receptors) induced high CBF (17.4 Hz). The administration of βE2B at a low (3.2 mg/kg/day) or high (32 mg/kg/day) dose decreased the CBF to 14.5 Hz or 11 Hz, respectively. ICI-182,780 abolished the βE2B-induced changes in CBF and decreased CBF to 12 Hz. The administration of mPRG (6.4 mg/kg/day) decreased CBF to 12.5 Hz. Mifepristone abolished this mPRG-induced decrease in CBF and maintained the CBF at 15 Hz. However, administering both βE2B and mPRG increased CBF to 17.5 Hz, suggesting that βE2B inhibits mPRG actions and vice versa. To confirm the interactions between βE2B and mPRG, we administered both βE2B and mPRG to guinea pigs that were pretreated for 1.5 days with either mPRG (6.4 mg/kg/day) or βE2B (3.2 mg/kg/day). Prior treatment with βE2B or mPRG prevented the increase in CBF that was otherwise by βE2B plus mPRG, and maintained the CBF at 14.5 Hz or 13 Hz, respectively. The administration of βE2B plus mPRG still induced the expression of PRG receptors, indicating that the highest CBF is not the result of no expression of the receptors. In the beating cilia of the fimbria, the signals that are activated by the βE2 and PRG receptors are proposed to antagonize each other in regulating the frequency.