Mastitis is the most common disease affecting dairy herds. The influence of mastitis on the oocyte and on early embryonic development is unclear. The aim of the present study was to examine the effects of endotoxin on oocyte developmental competence. Six lactating Holstein cows were synchronized using the Ovsynch protocol. PGF_2α was administered on Day 6 after the second GnRH injection, and 36 h later, either Escherichia coli endotoxin (LPS, 10 μg, n = 3) or saline (n = 3) was administered to one udder quarter of each cow. Milk samples were collected and rectal temperatures were recorded after treatment. Cows treated with LPS showed a typical increase in body temperature (40.27°C ± 0.37), whereas cows treated with saline did not (38.9°C ± 0.04). A higher (P < 0.05) somatic cell count was noted for cows treated with LPS. At the acute phase (6 h after LPS administration), which is characterized by increased secretion of cytokines and other substances, plasma samples were collected, and follicular fluid (FF) from the preovulatory follicle was aspirated by transvaginal probe (Pie Medical, Maastricht, The Netherlands) to serve as a maturation medium for IVF procedures. In 2 different experiments, bovine oocytes aspirated from ovaries collected from a local abattoir were matured (22 h, 5% CO₂, 38.5°C) in the indicated media, fertilized (18 h), and cultured for 8 days (KSOM, 5% CO₂, 5% O₂, 38.5°C). The proportion of oocytes that cleaved and developed to blastocysts was recorded 44 h and 7 to 8 days postfertilization (p.f.). In the first experiment, oocytes were matured in standard maturation medium (TCM-199, control), in FF aspirated from cows treated with saline (FF-saline) or in FF from cows treated with LPS (FF-LPS). Cleavage rate to the 2- to 4-cell stages did not differ among groups. However, the proportion of developed blastocysts in the FF-LPS group was lower than that in the FF-saline and control groups (10.6 v. 22.4 and 24.4%, respectively; P < 0.05). In the second experiment, oocytes were matured in standard maturation medium (control), plasma obtained from cows treated with saline (Plasma-saline), or plasma from cows treated with LPS (Plasma-LPS). Similar to the first experiment, cleavage rate did not differ among groups, and the proportion of blastocysts for the Plasma-LPS group on Day 7 p.f. was lower than that for the Plasma-saline and control groups (11.0 v. 25.5 and 34.7%, respectively; P < 0.05). A slight increase in blastocyst formation was noted on Day 8 p.f. in both the FF-LPS and Plasma-LPS groups (16.9 and 14.8%, respectively). Nevertheless, it remained lower (P < 0.05) than that in the control, FF-saline, and Plasma-saline groups (30.0, 27.3, and 28.7%, respectively). Although the mechanism is not clear, the findings suggest that endotoxin has deleterious effects on the ovarian pool of oocytes. Whether this is also the case for natural clinical mastitis remains to be examined.