The essential oil of Juniperus sabina, containing 50% sabinyl acetate, was administered s.c. to mice at doses of 15, 45 and 135 mg/kg either on days 0–4 of pregnancy (implantation period) or on days 8–11 (organogenesis) to investigate its mechanism of action. The mice were killed on days 9 and 17 respectively. Subsequently to the previous assays, sabinyl acetate was injected s.c. at a dose of 70 mg/kg, corresponding to the higher dose of essential oil, on days 0–4, the mice being killed on day 9. The number of prominent corpora lutea and implantation sites were recorded for mice killed on day 9. The uterine content of those killed on day 17 were examined and the fetuses removed and examined for malformations. Treatment with the oil on days 0–4 prevented pregnancy in half the mice treated with the two higher doses. The same pattern was observed with sabinyl acetate (70 mg/kg). In contrast, treatment with the oil on days 8–11 did not impair fertility, nor did it increase the incidence of malformations compared with the results obtained for the control group. This result indicates that the abortifacient effect of Juniperus sabina essential oil is related to an implantation inhibiting effect, induced by sabinyl acetate, the mechanism of which is being investigated.