Blood samples were taken from 11 cows and their ovaries were scanned by ultrasound at least daily. Around day 5 of an induced cycle, they were injected with 10 pg buserelin, an analogue of gonadotrophin releasing hormone, and on day 12 they received 0.5 mg cloprostenol, an analogue of prostaglandin F2α (PGF2α). Two days later six of the cows (the treated group) received a second injection of 10 μg buserelin, but the remaining fve received no further treatment (control group). The dominant, that is, the largest follicle in each cow disappeared after the first buserelin injection and was replaced by a new one which grew synchronously in all the cows until after the treatment with PGF2α. Ovulation occurred significantly earlier after PGF2α in the treated group than in the control group (72 to 96 hours v 96 to 120 hours; P<0.05). Plasma progesterone concentrations then increased more rapidly in the treated group than in the control group and were significantly higher on days 3 and 4 after ovulation (P<0.05).