1. The effect of colchicine, vincristine and griseofulvin (GRIS) on the porphyrinogenic action of 2-allyl-2-isopropylacetamide (AIA) and veronal was studied in vivo and using the in vitro experimental model of tissue explant cultures. 2. Complete prevention by colchicine was found in liver and heart explant from animals treated with AIA and veronal. 3. Vincristine, GRIS and colchicine reversed AIA induction in liver explants, however reversal was partial or nil in skin and heart explants depending on the antimitotic and the tissue. 4. The usefulness of the combination of the in vivo experimental model and the in vitro explant tissue culture model, for this kind of studies is emphasized.