Measurement of extracellular fluid volume (ECFV, haemolymph or blood volume) of abalone is important for understanding respiratory and circulatory functions and for investigation of factors affecting meat recovery in the fishery. The compounds ¹⁴C-inulin and ⁵¹Cr-ethylenediaminetetraacetic acid (EDTA) were evaluated as markers for the determination of ECFV and primary urine formation (filtration) in New Zealand blackfoot abalone or paua, Haliotis iris. The volume of distribution of either marker reliably estimated ECFV after correction for clearance (56.6 ± 3.5 mL 100 g^–1 soft tissue). ⁵¹Cr-EDTA was cleared from the haemolymph three to five times faster than ¹⁴C-inulin and was sequestered in the kidneys, digestive gland and mucous gland. ⁵¹Cr-EDTA was therefore unsuitable as a filtration marker. Urine filtration rate was best estimated from the appearance of inulin in the seawater (16.2 ± 2.1 mL 100 g^–1 soft tissue day^–1). The ECFV (inulin space) of isolated tissues ranged from 12–17 mL 100 g^–1 tissue in the digestive gland, foot and adductor muscle to 60–70 mL 100 g^–1 tissue in the kidneys, right ctenidium and epipodium. Equalisation of both markers between the efferent ctenidial vein and haemolymph lacunae of the shell adductor muscle occurred extremely slowly (t_1/2 ~2 h, t_0.95 ~8 h), confirming the poor perfusion of this anaerobic muscle.