Five cows, diagnosed as having follicular cysts (by palpation per rectum, ultrasonography and progesterone determination), were treated with 0.5 mg gonadotrophin releasing hormone (GnRH) 10 days after initial diagnosis. The ovaries were scanned weekly from the day of treatment (day 0) until day 35. Mean (+/- SEM) plasma progesterone concentration on the day of diagnosis (day -10) and day 0 were 0.2 +/- 0.13 ng ml-1 and 0.72 +/- 0.27 ng ml-1, respectively. Elevated progesterone (5.58 +/- 0.67 ng ml-1) was found in all cows by day 7 after treatment. Changes noted during weekly ultrasonography included clouding of the uniformly nonechogenic (dark) antrum of cysts, luteinization of the cyst wall, reduction in cyst size (cyst resolution) and/or development of 1-4 corpora lutea in the ovary bearing the cyst or in the contralateral ovary. Oestrus was observed in all cows 18.8 +/- 1.74 days after treatment. It is concluded that ultrasonography and progesterone determination are useful tools for veterinarians in the monitoring of responses of follicular cysts to treatment.