A theoretical compartment model for the uptake of Cd and Zn by the freshwater crayfish, Procambarus acutus acutus (Girard), was constructed, and included a factorially designed experiment to study the relative importance of food and water as uptake vectors for 109Cd and 65 Zn. Food and water were important pathways for Cd accumulation and the two uptake vectors were first order, independent, and additive. The rate of Cd elimination was not significantly different from zero during either the uptake or depuration phases. Rate constants for uptake from 5 and 10 micrograms Cd . L-1, calculated on a concentration basis, were 0.84 +/- 0.031 and 0.83 +/- 0.029 day-1, respectively. Although there was not measurable Cd elimination, crayfish reached a steady state concentration of about 12.5 micrograms Cd . g-1, dry weight. A small amount of Zn was accumulated via food, relative to that accumulated from water; the former had a much longer retention time than the latter, which was lost rapidly. When Zn in food was the only source, steady state was reached rapidly; Zn accumulation from food and water was not additive. Zn accumulated from water, reached no steady state. Zn uptake was proportional to water concentrations within the fed treatment groups and those not fed Zn-contaminated food. Zn elimination was first order to Zn concentration in the crayfish. Rate constants for Zn accumulation from 50 and 100 micrograms Zn . L-1 when fed Zn-contaminated food were 1.34 +/- 0.064 and 1.46 +/- 0.073 day-1 (estimate +/- 95% CI based on concentration), respectively, while rate constants for Zn elimination were 0.034 +/- 0.0021 and 0.05 +/- 0.0032, respectively.