The effect of various organic metal compounds on δ‐aminolevulinic acid dehydratase (ALAD, porphobilinogen synthetase) activity has been studied. Various organic tin and lead compounds have little effect on this enzyme. However, triethyl lead chloride has a potency similar to that of inorganic lead nitrate in inhibiting ALAD both forin vitrostudy and afterin vivodosing. Liver and blood ALAD have a similar sensitivity to lead compounds, which is reduced in the presence of zinc. Trimethyl lead chloride inhibits ALADin vitroto a lesser extent. The results suggest that amphiphilic organic lead compounds may directly inhibit ALAD without prior degradation to inorganic lead. The diffusibility and persistence of triethyl lead combine to make it an especially hazardous lead compound