A luteolytic dose (500 micrograms) of cloprostenol was given four days before expected estrus to five mature cyclic cows of native Taiwan Yellow cattle. Blood samples were collected at hourly intervals for 60 hours and then twice daily (0800 h and 1600 h) until onset of the following estrus or Day 29 after prostaglandin (PG) injection. Plasma progesterone levels were quite low in the follicular phase and elevated over an 11-13 day period during the luteal phase for every animal. Two cows showed peak luteinizing hormone (LH) levels of 19.97 and 23.77 ng/ml of plasma at 120 and 127 h after PG injection. One cow did not show peak LH level during the 60-h sampling period, but had its highest concentration of 18.02 ng/ml at 0800 h on Day 8. Two cows showed no detectable peak LH during the 60-h sampling period or on subsequent days. But these two cows had increased concentrations of progesterone beginning on Day 6 and Day 12. All animals came back into estrus 23-28 days after PG treatment.