Cystic ovaries were experimentally produced by i.m. injection of 5 mg oestradiol valerate on Day 16 of the oestrous cycle or by s.c. injection of 100 ml antiserum to bovine LH before the onset of oestrus. The length of the cycle after both treatments was altered. The cysts formed after oestradiol injection were relatively small (2·5-3 cm diameter) and thin-walled, and were accompanied by relatively low plasma oestrone and high oestradiol levels; those after LH antiserum treatment were large (5·0-6·0 cm) and firm and associated with relatively low oestradiol and high oestrone levels. The results of the hormone assays after treatment support the theory that cystic ovaries could result from premature LH release or an insufficiency of LH at the time of ovulation.