Net influxes into the haemolymph and tissue distribution of 45Ca and 109Cd were studied in vivo in female Carcinus maenas at different moult stages. Net influxes of 45Ca and 109Cd from water were higher in postmoult (A and B) C. maenas than in C3- and C4-intermoult crabs and the net influx of calcium was higher in C3-intermoult crabs than in C4-intermoult crabs. The net influxes of 45Ca and 109Cd increased in postmoult C. maenas with decreasing external calcium concentrations at constant salinity. At all external calcium concentrations a significant correlation existed between 45Ca and 109Cd accumulated in the haemolymph of individual animals. In vivo exposure of postmoult C. maenas to external lanthanum decreased the 45Ca and 109Cd uptake rates to 30 and 10%, respectively, of the control values. About 30% of injected 109Cd were found in the midgut gland, 10-20% in the gills and only a few (1-2) percent was lost to the seawater 24 h after injection. No major variations in tissue distribution of 109Cd were observed between moult stages in these tissues. Premoult crabs retained more cadmium in the haemolymph 24 h after injection than other moult stages, and postmoult crabs retained more in muscle. Between 20 and 40% of the injected 45Ca were excreted to the water, while only a few percent of the injected 45Ca were found in the soft tissues 24 h after injection. Large moult stage variations, however, were observed in the tissue distribution of internalised 45Ca. This study demonstrates that cadmium and calcium uptakes are elevated in postmoult C. maenas. The results indicate that cadmium and calcium in this stage are taken up via Ca2+-channels located in the apical membrane of gill epithelium cells. When internalised, however, cadmium and calcium are metabolised in fundamentally different ways, determined by the chemical properties and biological significance of the two metals.