This study assessed the value of ultrasonography in characterising bovine cystic ovaries and monitoring their responses to different treatments. Thirteen cows were diagnosed by ultrasonography as having luteinised ovarian cysts and seven were diagnosed as having follicular ovarian cysts. Six of the former were treated with prostaglandin, four with a progesterone intravaginal device (PRID) and three with gonadotrophin-releasing hormone (Gn-RH); five of the latter were treated with Gn-RH and two with a PRID. All the animals were re-examined by ultrasound and blood was collected for the measurement of plasma progesterone concentration at intervals until oestrus. The treatment of the luteinised cysts with prostaglandin caused marked decreases in size and plasma progesterone concentration and altered their echotexture within two to four days; oestrus occurred within three to four days. In two of the cows treated with a PRID the cysts regressed within one to two weeks but the other two cows required supplementary treatment with prostaglandin; oestrus and ovulation were observed only after the cysts collapsed. Gn-RH stimulated oestrus and ovulation within three to four days but the cysts did not collapse until much later. The treatment of the follicular cysts with Gn-RH or a PRID caused fresh ovulation and the formation of a corpus luteum but had little immediate effect upon the cyst. The plasma progesterone concentrations in some of the cows with either follicular or luteal cysts were similar on the day of treatment and were therefore of little value in differentiating the types of cyst.(ABSTRACT TRUNCATED AT 250 WORDS)