Sediments often constitute the major repository of metals and may be a potential source for metal bioaccumulation by marine deposit- and suspension-feeding invertebrates. In this study, we compared the uptake of Cd, Cr, and Zn from solute and sedimentary sources by a deposit-feeding peanut worm, Sipuncula nudus. The uptake rate constants determined for the three metals were generally low and were 0.0016-0.0020 l g(-1) d(-1) for Cd, 0.0107-0.0269 l g(-1) d(-1) for Cr, and 0.0235-0.0463 l g(-1) d(-1) for Zn. The uptake rate of Zn increased at a disproportionately slower rate with increasing Zn concentrations in the ambient water, indicating that Zn may have been partially regulated at a high Zn concentration. The assimilation efficiency (AE) of the metals was determined using a pulse-chase radiotracer technique. The AEs were in the range of 6-30% for Cd, 0.5-8% for Cr, and 5-15% for Zn. The sediment grain size and radiolabeling duration (between 7 and 30 d) did not affect the metal AE. There was no major difference in the metal AE from natural sediment collected from a contaminated environment. The desorption of metals from the radiolabeled sediments was also concurrently measured using the gut juice extracted from the sipuncula. Up to 63% of Cd was extracted by the gut juice, whereas only <4% of Cr was extracted by the gut juice. There was a significant relationship between the metal AE and the metal extraction by gut juice for Cd, whereas no significant relationship was found for Cr and Zn. Thus, the bioavailability study using the gut juice extraction method may indicate Cd bioaccumulation in the sipuncula. With a simple kinetic model, we show that the bioaccumulation of Cd, Cr, and Zn in the worms is largely dominated by sediment ingestion due to the low uptake from the solute phase as well as the high metal concentrations in the sediment. Thus, sediment should be considered as a direct source for metal accumulation in sipuncula when setting the sediment quality criteria.