Abstract Tests were made of γ-emitting compounds as potential non-absorbed reference markers for estimating water ingestion in the western rock lobster Panulirus longipus cygnus (George) and the penaeid prawn Penaeus latisulcatus (Kishinouye). The extent of any marker absorption into the tissues was measured in animals totally immersed and in rock lobsters with only the gills perfused. Distribution of marker within the body, transport into the gut, and excretion were examined following injection of the label into the blood of the rock lobster. 125I-sodium iothalamate, 58Co-EDTA, and 51Cr-EDTA were all satisfactory reference markers with very low absorption, 51Cr-EDTA being the best. Most injected sodium iothalamate was excreted from the body over 24 h, probably via the urine, which suggests its further use in studies of antennal gland function. 46ScCl 3, 51CrCl 3, 58CoCl 2, and 110AgCl were of some value for estimating drinking in penaeid prawns, although 110AgCl probably gave levels that were too high. 110AgCl was useless for the rock lobster, which absorbed much more marker than was retained in the gut. 125I-rose bengal was not practicable because it slowly precipitated in sea water. Requirements for reference markers appear to be more critical for animals with high permeability to major sea-water ions than those which extensively regulate all major ions.