The milk progesterone test; a means to improve fertility in dairy herdsThe blood progesterone concentration, depending on corpus luteum activity, is reflected by the amount of progesterone in the milk. Since this was discovered in the seventies, dairy farmers and veterinarians are utilizing this fact by having progesterone concentrations determined in milk samples with the purpose of recognizing the respective physiological or pathological state of a cow's ovaries. The practicability of the test is facilitated by the recent availability of quick, cheap methods for determining milk progesterone concentrations.The test may be applied by dairy farmers to examine the proper time of insemination, check for cyclicity and recognize return cows. More complex physiological or pathological ovarian conditions are usually monitored by analysing progesterone concentrations in a sequence of samples usually collected under supervision of a veterinarian. In this context the milk progesterone test may serve as a valuable diagnostic tool to recognize conditions such as silent heat, lack of cyclicity, cystic follicles, retained corpora lutea and irregular cycles. It may also serve to monitor the ovarian response to therapeutical or biotechnical measures or to improve the accuracy of fertility parameters.