The objective was to characterize the effects of Escherichia coli lipopolysaccharide (LPS) endotoxin (given i.v.) on luteal structure and function. Seven nonlactating German Holstein cows, 5.1±0.8 years old (mean±s.e.m.), were given 10 ml saline on day 10 (ovulation=day 1) of a control estrous cycle. On day 10 of a subsequent cycle, they were given 0.5 μg/kg LPS. Luteal size decreased (from 5.2 to 3.8 cm(2), P≤0.05) within 24 h after LPS treatment and remained smaller throughout the remainder of the cycle. Luteal blood flow decreased by 34% (P≤0.05) within 3 h after LPS and remained lower for 72 h. Plasma progesterone (P(4)) concentrations increased (P≤0.05) within the first 3 h after LPS but subsequently declined. Following LPS treatment, plasma prostaglandin (PG) F metabolites concentrations were approximately tenfold higher in LPS-treated compared with control cows (9.2 vs 0.8 ng/ml, P≤0.05) within 30 min, whereas plasma PGE concentrations were nearly double (P≤0.05) at 1 h after LPS. At 12 h after treatment, levels of mRNA encoding Caspase-3 in biopsies of the corpus luteum (CL) were increased (P≤0.05), whereas those encoding StAR were decreased (P≤0.05) in cattle given LPS vs saline. The CASP3 protein was localized in the cytoplasm and/or nuclei of luteal cells, whereas StAR was detected in the cytosol of luteal cells. In the estrous cycle following treatment with either saline or LPS, there were no significant differences between groups on luteal size, plasma P(4) concentrations, or gene expression. In conclusion, LPS treatment of diestrus cows transiently suppressed both the structure and function of the CL.