Thirty four heifers were estrus synchronised by two injections of PGF2a at an 11 day interval. Clinical and ultrasonic examinations of ovaries were performed daily. Ovulation occured in 91% of the animals within 6 days after the second prostaglandin injection. Heifers ovulated in 4 day period with the greatest intensity on the 3rd day after PGF2a treatment. Ovulation of the greatest follicle present on ovaries was discovered in 80% of the animals at the time of luteolysis induction. In 1 heifer double ovulation was observed. It was found that the growth of ovarian follicles immediately before the second injection of PGF2a was asynchronic. Fixed time (72 and 96 h) of insemination gives no chance of conception in about 25% of ovulating animals.