Hepatic porphyrias (acute intermittent porphyria, coproporphyria, porphyria variegata and porphyria cutanea) are inherited diseases affecting porphyrin-heme metabolism in the liver. A situation analogous to the latent stage of human hepatic porphyria can be produced in chick embryos by injecting an inhibitor of ferrochelatase (the last enzyme in the heme biosynthesis pathway); the simultaneous injection of a porphyrinogenic drug (e.g. barbiturate) results in a strong increase of liver porphyrin levels, as is found in human hepatic porphyrias. This experimental model facilitates drug screening: drugs that are found to be porphyrinogenic in ovo are potentially dangerous to patients with hepatic porphyria and should be avoided.