Thirteen cows received 0.75 mg tiaprost by subcutaneous injection during the mid-luteal phase of the ovarian cycle. Twelve underwent rapid luteolytic and continued to cycle normally as determined by milk or plasma progesterone concentrations. The other cow underwent only a transient decrease in progesterone concentrations. A further 15 cows received either a subcutaneous osmotic mini-pump containing 30 mg tiaprost , calibrated to release 0.75 mg over a 24 hour period (group 1); a subcutaneous injection of 0.375 mg tiaprost (group 2), or a subcutaneous injection of 0.375 mg tiaprost plus the osmotic mini pump (group 3). Treatment caused luteolysis in zero and one cows in groups 1 and 3 respectively. However all six cows in group 2 underwent rapid luteolysis. Although the standard therapeutic dose of tiaprost (0.75 mg) appears to be a highly effective luteolytic agent it is concluded that there may be scope for development of longer acting formulations of this compound.