After screening 269 crude drugs for their ability to inhibit the development of mouse zygotes, we found Coptis rhizome and Phellodendron bark to have inhibitory effects. We examined the effects of both extracts and of berberine, a major component of these plants, on in vitro development of zygotes and on fullterm fetal development in the mouse. Mouse zygotes were cultured in medium containing water-soluble extracts of Coptis rhizome or Phellodendron bark, or berberine at various concentrations for 5 days and the potential of zygotes to develop to blastocysts was examined. In addition, superovulated mice were intramuscularly injected with berberine and mated, and examined for the in vivo development of fertilized eggs to blastocysts and full-term fetuses. In vitro development of zygotes to blastocysts was almost completely inhibited when they were cultured in medium containing more than 0.1 µg/ml Coptis rhizome, 10 µg/ml Phellodendron bark, or 0.01 µg/ml berberine chloride or berberine sulfate. When superovulated and mated females received 100 µg berberine chloride once a day for 2 to 14 days, the proportions of recovered blastocysts and full-term fetuses were significantly decreased. The present study indicates the potential use of berberine as a contraceptive for animals.