In order to overcome the refractory period of the corpus luteum to prostaglandin (day 15 of the cycle through oestrus to day 3 of the next cycle), two injections each consisting of either 31,25 μg; 62,5 μg; 125 μg or 250 μg of cloprostenol were injected at a 10-day interval. An increase in the dosage was followed by a significant increase in oestrous response (50,0 per cent; 56.3 per cent; 81.3 per cent and 100.0 per cent). Higher dosages (125 μg and 250 μg) of cloprostenol caused more rapid and more complete luteolysis as reflected by the decrease in the plasma progesterone concentration, while lower dosages (31 25 μg and 62,5 μg) often failed to induce complete luteolysis.