Levels of lh were determined by radioimmunoassay and those of progesterone by a competitive protein-binding technique. Blood samples from eight cows, representing two complete pregnancies (including post-partum periods), and from one cow in the first trimester and another around the time of parturition, were collected. The samples were withdrawn at intervals of 6 hr for lh and every 4 to 5 days for progesterone determination. Levels of lh were consistently low (1·0 to 1·6 ng/ml plasma) with a few single peaks occurring irregularly in individuals during the first 110 days. Distinct lh peaks were observed between Days 7 and 20 post partum. The decrease of progesterone from high levels during pregnancy (plateau between 5 and 9 ng/ml plasma with individual differences) before parturition and the low level following was not reflected in the lh concentration. In an experiment with one animal, an injection of 5 mg flumethasone near term caused a partial decrease in the progesterone concentration, and a larger dose of 10 mg induced parturition. This was preceded by a marked decrease of progesterone similar to that observed before a normal delivery.