Avian, amphibian and fish embryos were given continuous treatment with inorganic mercury, methyl mercury, cadmium and lead, to determine the sensitivity of embryogenesis to metallic poisoning. All metals produced substantial degrees of lethality and/or gross anatomical anomalies at 10 ppb or less. Treatment with inorganic mercury at 10 ppb produced 100% kill of frog embryos. Chick and rainbow trout embryos suffered 10-20% lethality when exposed to 1 ppb of either inorganic or methyl mercury. Lead and cadmium at 1 ppb produced 24-32% lethality in chick embryos. No significant differences were observed in the embryopathic effects of inorganic or methyl mercury. Concerning the toxic effects of mercury, cadmium and lead, the 'embryonic stage' appears to constitute the critical 'sensitive link' in the vertebrate life cycle. The reporductive potential of vertebrate populations may be severely restricted (e.g., embryonic mortality) by such pollutants at trace levels which may not prove hazardous to adult animals.