Twenty-four regularly cycling female CHI mice were exsanguinated at known points in the estrous cycle (as determined by vaginal smear). Their reproductive tracts were removed and examined histologically to confirm the phase of the cycle. Plasma samples from each animal were bioassayed for “free” progestin, and the results, expressed in Progesterone Equivalents (P.E.), were examined in relation to the phases of the cycle. It was noted that the plasma progestin content is at a minimum (0.5 P.E./ml.) in diestrus, rises to a maximum during proestrus and estrus (average about 4 P.E./ml.), and then decreases in metestrus–1 to approach the minimum in metestrus–2. The mouse is thus added to the list of animals secreting progestin before ovulation.