This manuscript focuses on potential changes in reproductive physiology that occur due to high milk production in lactating dairy cows. Four reproductive measures are discussed: interval to first ovulation, conception rate, duration of estrus, and multiple ovulation rate. The last two responses have now been closely linked to level of milk production. In contrast, time to first ovulation does not appear to be associated with level of milk production, and the association of conception rate with level of milk production is still controversial. In an attempt to explain some of the changes in reproductive physiology caused by high milk production a model of elevated steroid metabolism in lactating dairy cows is presented. Although many aspects of this model remain to be tested, a central role for elevated steroid metabolism in lactation-induced reproductive changes seems likely. © 2005 Elsevier Inc. All rights reserved.