Following parturition blood samples were collected daily from 5 mature lactating cows until the first post partum oestrous period. Plasma progesterone concentration as determined by RIA was applied to evaluate post partum ovarian activity. Parturition to first oestrus varied from 57-77 days between individuals. During the major part of this period. plasma progesterone concentration fluctuated between 0,4 and 1,6 ng/ml. Although an increase in plasma progesterone concentration prior to first oestrus was observed in all the cows, the occurrence of a silent ovulation was evident in only one of the cases. In this animal plasma progesterone levels exhibited the same pattern as described during the luteal phase of a normal oestrous cycle. In the 4 remaining cows progesterone levels increased from 0,5-0,7 ng/ml,6-8 days before oestrus to 2,1-3,0 ng/m1, 2-3 days before oestrus, whereafter it dropped to 0,3-0.7 ng/ml at oestrus. The duration of these plasma progesterone increases lasted 4-5 days as compared to 20 days following a silent ovulation.