Plasma progesterone levels were determined throughout pregnancy (day of vaginal plug = day 0) and on the day of parturition by a competitive protein-binding assay technique. Nulliparous females of a random-bred stock derived from a four-way cross were maintained on a 14-hr light schedule (on between 0500–1900 hr) and were sacrificed between 0900–1000 hr. Progesterone was low on day 0 (9.9 ng/ml) rising to a plateau of between 41 and 54 ng/ml on days 2 to through 9. A significant drop (p < 0.001) in progesterone to 35.5 ± 1.4 ng/ml occured on day 10. Progesterone concentrations then rose again to a peak level of 112.7 ng/ml on day 15. This was followed by a continuous decline to 5.5 ng/ml on the day of parturition. (Endocrinology94: 1209, 1974)