We have previously demonstrated that the small metal-binding protein, metallothionein (MT), plays an important role in the metabolism of Cu and Zn during the molt cycle of the blue crab, Callinectes sapidus. To further delineate the role of MT in the regulation of both metals, the distribution of copper and zinc was examined immediately after ecdysis in the blue crab. Hemolymph, digestive gland, and stomach were analyzed, by atomic absorption spectrophotometry (AAS), for total metal concentration in crabs at different molt stages, from premolt (D3) through paper shell (B2), and including intermolt (C4). Cytosolic extracts were prepared from digestive glands of individual crabs and analyzed, by gel filtration chromatography and AAS, for MT, copper, and zinc. The short-term changes in metal concentrations in the tissues, and those in MT and metals in the cytosol were dramatic. Transient changes in the metals bound to MT correlated well with the loss of copper from the hemolymph and the digestive gland. The observed changes occurred over a period of 90 min after ecdysis. The data suggest that copper is stripped from hemocyanin in the digestive gland after ecdysis, displacing zinc from MT in the cytosolic pool. We hypothesize that the copper/zinc-MT complex may then be sequestered in lysosomes and eliminated into the gut and out in the feces. A discriptive flow model showing the involvement of MT in copper and zinc partitioning after ecdysis in the blue crab has been constructed.