An analysis on the outcome of 124 inseminations monitored using daily milk progesterone concentration data revealed that those cows in which pregnancy did not occur experienced a 1.7day delay in post-ovulatory luteal progesterone rise when compared to the pregnant group (P<0.001). This suggests that luteal progesterone secretion needs to be initiated prior to day 5 post-ovulation. A comparison between the milk progesterone concentration on day 5 post-ovulation and the pregnancy rate for 1451 inseminations showed the maximum pregnancy rate to occur when the milk progesterone concentration was between 3-9ng/ml. Pregnancy rate fell significantly (P<0.01) when progesterone concentrations were below 3ng/ml, identifying cows that may benefit from progesterone supplementation. The supplementation of progesterone in cows with a day 5 milk progesterone concentration of less than 3ng/ml, using an intra-vaginal progesterone releasing device, only provided benefit for those cows with a day 5 progesterone concentration of between 1-2ng/ml, resulting in a doubling of the pregnancy rate. This data shows that a delay in post-ovulatory progesterone secretion has a detrimental effect on pregnancy but that not all cows with a deficient progesterone concentration benefit from progesterone supplementation.