Although little information is available on the relative blood volumes in individual tissues of fish, such information is often indispensable in modelling the uptake and distribution of chemicals in the body. To elucidate metal uptake by aquatic organisms, an accurate determination of the branchial metal accumulation rate is required. Because it is not always possible to remove all the blood from the gills, it is necessary to correct the branchial accumulation rate results for the amount of metal ions trapped in the blood. In this article, a method to provide an estimate of gill blood volume to enable subsequent volume corrections is described. One of the quench indicating parameters (the transformed spectral index of the external standard), provided by a liquid scintillation analyser, is used to equate the amount of spectral shift in quenched gill tissue to the trapped blood volume. It is shown that the shift and absorbance at 413 nm are closely correlated and that gill tissue itself has only a small effect on the spectral shift and absorbance. A correction factor was derived to account for this tissue effect. It is shown that the method presented gives reliable and consistent results. The method has been used to study the effect of organic complexation on the transport of cobalt into (fish) gill cells.