Abstract 1. The effect of cadmium, lead, and mercury on 5-aminolevulinic acid (ALA), porphobilinogen (PBG), and PBG synthase was determined in hepatopancreas of the bivalve, Cerastoderma edule (L.). 2. Cd and Hg exposure induced increased ALA content, and thus an initial doubling of ALA within 24 hr. 3. Using ALA in excess (8 mmoll −1) as substrate, no PBG synthase (ALA dehydratase, EC 220.127.116.11) activity was detectable in freshly prepared hepatopancreas homogenates. 4. Increased ALA in metal exposed bivalves is not a simple effect due to metal inhibition of PBG synthase. 5. The observed lack of PBG synthase suggests an alternative to the general pathway where two ALA molecules condense to one PBG.