Marine gastropods are known to accumulate high metal concentrations in their tissues, especially in the digestive glands. Although bindings with metallothioneins and granules are proposed to be responsible for the accumulation of high metal concentration, routes and rates of metal accumulation in these animals are not known. In this study, we determined the biokinetics of Cd and Zn in two predatory marine gastropods (Babylonia formasae habei and Nassarius teretiusculus), including metal assimilation efficiency (AE) from ingested clam tissues, metal uptake rate from the dissolved phase, and metal efflux rate constant. The metal AEs were 87-99% for Cd and 50-79% for Zn in B. formasae habei, and were 84-94% for Cd and 79-81% for Zn in N. teretiusculus, respectively, feeding on clams radiolabeled from both the aqueous and dietary phases. The uptake rate constants from the dissolved phase were 0.0562 l g(-1) per day for Cd and 0.122 l g(-1) per day for Zn in N. teretiusculus, and 0.0292 l g(-1) per day for Cd and 0.0573 l g(-1) per day for Zn in B. formasae habei, respectively. The efflux rate constants were 0.0011 per day for Cd and 0.0138 per day for Zn in N. teretiusculus, and 0.0055 per day for Cd and 0.0057 per day for Zn in B. formasae habei, respectively. Using a simple kinetic model, we showed that the trophic transfer factor, defined as the ratio of metal concentration in predatory gastropods and metal concentration in prey organisms (bivalves), was greater than one under most circumstances, suggesting that Cd and Zn may be biomagnified during their trophic transfer from bivalves to gastropods. Furthermore, our model predicted that the dietary exposure dominated (>90%) the overall Cd and Zn accumulation in the two predatory gastropods. Our study also highlights the interspecies difference in metal biokinetics in marine gastropods.