The uptake and loss of 65Zn by Pacific oysters are fitted by solutions to a first-order, linear differential equation that describes “single-compartment” isotope substitution kinetics. The rate constants for loss are related to the rate constants for uptake and to steady-state concentration factors for stable zinc and 65Zn. The single-compartment solutions adequately fit uptake and loss data from field experiments extended as long as several hundred days. The rate constant estimated from loss data is 5.49 × 10−3 d−1 and that from uptake data is 4.93 × 10−3 d−1. The apparent importance of multiple compartments can be affected in experiments where uptake times are short.