Sublethal doses of cadmium chloride (2.5, 5.0 and 10.0 mu mole/kg egg weight) were found to significantly alter the first two rate limiting enzymes of heme biosynthesis in chick embryos. Delta-aminolevulinic acid synthase activity was elevated by 2.05 and 2.11 fold with 5.0 and 10.0 mu moles of cadmium treatment respectively. However, this was reduced to 1.25 and 1.3 fold by the simultaneous administration of ascorbic acid. Blood delta-aminolevulinic acid dehydratase (ALA-D) activity was decreased by 48.4 and 55.0 per cent with 5.0 and 10.0 mu moles cadmium treatment respectively; in the presence of ascorbic acid only 18 and 24 per cent inhibition of ALA-D activity was observed. Further 1.39 and 2.08 fold accumulation of delta-aminolevulinic acid and 4.17 and 4.62 fold increase of blood porphyrins was observed in chick embryos treated with 5.0 and 10.0 mu moles cadmium respectively. This elevation of intermediate compounds of heme biosynthesis was effectively checked by the administration of ascorbic acid. Depletion of hepatic heme and free sulfhydryl level by cadmium were countered by the treatment of ascorbic acid. Hence, the present findings suggest the protective role of ascorbic acid against cadmium induced chemical porphyria in chick embryos.