A hanging mercury drop electrode, included in a low deal volume thin layer flow cell, has been used for on‐line simultaneous determination of the trace metals Cd, Pb, Cu and Zn. Parameters related either to the flow system or to the differential pulse anodic stripping technique were optimized. Internal standard and standard additions quantitation methods were tested. Bi was chosen as internal standard and adequate conditions for the stripping medium were studied. The method was applied to the determination of these metals in algae samples of the north coast of Spain. The results compared favorably with those obtained by ICP‐MS.