Changes in lipid contents and fatty acid compositions of each lipid fraction were examined in corpora lutea from 34 unmated ewes between Days 8 and 16 of the estrous cycle and from 6 ewes at Day 16 of pregnancy. Four patterns were observed during advancement of the estrous cycle. Luteal concentrations of free cholesterol and triglyceride (neutral lipids) increased between Days 14 and 16, during luteal regression, in a manner approximated by exponential functions of time, whereas luteal concentrations of phospholipid (polar lipids) increased and then decreased between Days 8 and 16 in a manner approximated by a sin function of time. Likewise, within the various lipid class component fatty acids, changes in palmitic acid weight percentages were approximated by sin functions of time, whereas arachidonic acid weight percentages increased between Days 14 and 16 in a manner approximated by exponential functions of time. Pregnancy either inhibited or reversed the changes in luteal lipid profiles, especially arachidonic acid percentages, between Days 14 and 16 of the estrous cycle. Luteal lipid profiles of corpora lutea from Day 16 pregnant sheep approximated lipid profiles of corpora lutea recovered from sheep between Days 12 and 14 of the estrous cycle. Comparison of luteal lipid profiles after tissue incubations at either 0 or 37 degrees C for 2 h revealed an effect of reproductive status on fatty acid metabolisms at Day 16. Changes observed in luteal lipid contents and fatty acid compositions during advancement of the estrous cycle represent aspects of lutein cell maturation and impending senescence that can be inhibited or reversed by pregnancy.