Feeding ruminants with Pittosporum undulatum can adversely affect the immune, endocrine, and reproductive systems. The aim of the present study was to evaluate how feeding with P. undulatum alters progesterone levels during the oestrus cycle, and fertilization and development of oocytes after in vitro fertilization (IVF). Heifers were fed a diet containing (n = 4) or not containing (n = 4) this plant. Peripheral blood samples were collected every 3 d and progesterone levels were analyzed by enzyme-linked immunosorbent assay (ELISA). The heifers’ ovaries were recovered after slaughter, and the oocytes were removed, maturated, in vitro fertilized, and cultured for 7 d, during which time their development was evaluated every 2 d. Results indicated that plasma concentrations of progesterone during the oestrus cycle were significantly higher (P < 0.01) in the luteal and follicular phases of the animals fed without Pittosporum when compared with those fed with Pittosporum. Statistical differences (P < 0.05) in maturation, cleavage rate, and embryonic development were observed between oocytes belonging to the experimental and control group. Rates of maturation, fertilization, and embryo development in the experimental group were, respectively, 78.3 ± 5.8, 29.92 ± 4.31, and 7.30 ± 3.1 compared with 90.5 ± 3.0, 41.86 ± 5.58, and 21.88 ± 6.85 in the control group. These differences can be explained because compounds from this plant can directly or indirectly inactivate cyclooxygenase-2 (COX-2), which may diminish follicular development through the inhibition of prostaglandins synthesis and oocyte maturation and, consequently, reduce the ability of oocytes to be fertilized and developed.