Previous investigators have demonstrated that 2.5 mg fo progesterone, administered intramuscularly to rabbits on the day of ovulation and the 2 preceding days (Days -2, -1, and 0) significantly and consistently accelerates ovum transport. In contrast, when given on the day of ovulation and the 2 following days (Days 0, +1, and +2), progesterone does not accelerate ovum transport. The experiments reported were designed to define more precisely the temporal relationships critical to progesterone-induced acceleration of tubal ovum transport. Our observations suggest 3 important conclusions: 1) Progesterone, when given at least 1 day, and not more than 2 days, prior to ovulation does induce accelerated ovum transport. 2) The progesterone responsive mechanism is dose dependent. 3) The acceleration is partially antagonized if progesterone treatment is begun 3 days prior to ovulation.