The effects of prostaglandin E2 (PGE2) on cervical opening in non-pregnant and pregnant heifers was studied, and in the pregnant animals the effect on the embryo was studied by means of ultrasonography. In four consecutive experiments, 5 ml of saline, a gel containing 2 mg or 6 mg PGE2 was administered intracervically to four non-pregnant heifers, and 2 mg of PGE2 was administered to heifers pregnant 33 to 40 days. All the groups treated with PGE2 experienced an increase in the concentrations of prostaglandin metabolite in plasma (P < 0.05) shortly after administration, which reached a peak 15 to 30 minutes after administration. An increase in cervical opening was evident in all the PGE2-treated heifers (P < 0.05) from three hours after treatment. There was no difference between the effect of the two doses. The heifers which received saline did not show any significant changes. In addition, the treated heifers showed cervical softening, congestion and mucus secretion which were more pronounced in the pregnant heifers. The embryos were not affected. Plasma progesterone concentrations remained unchanged in all the experiments.