Sequential blood samples were collected at 3 h interval from 3 Swedish dairy cows starting from the day of first post-partum ovulation for 10 consecutive days to describe short luteal phases. All plasma samples were analysed for the concentrations of the main PGF2 alpha metabolite, 15-ketodihydro-PGF2 alpha, whereas levels of progesterone were monitored from all morning samples. The day of ovulation was judged when the largest follicle, being monitored by a real-time B mode ultrasound scanner, could not be detected at the next examination. A sustained rise above 0.5 nmol/l of progesterone level was taken as a clear-cut value between non-luteal and luteal phases. Luteal phases of less than 8 days were registered as a short luteal phase during which the cows showed a total of 8 to 11 significantly elevated levels of the prostaglandin metabolite. The number of the significant increases of the metabolite was calculated using a skewness method. Analysis of these significant increases showed the first 1 to 4 episodes without altering the concentrations of progesterone. This would suggest that the developing corpus luteum is refractory in the beginning and thus, to induce luteolysis several PGF 2 alpha releases are required. The magnitude of progesterone concentrations during the short luteal phase is lower than the following phases.