The present study used fluorescent microspheres to examine the circulation in Haliotis iris. Fluorescent microspheres have not been used previously to study the circulatory system of invertebrates, despite their extensive application in the field of vertebrate circulatory physiology. A method of recovery and quantification of injected microspheres from the organs of H. iris was employed and refined. Satisfactory recoveries of the injected labels using the modified recovery method were achieved (>90%), and the relative distribution of the blood flow to the organs in resting animals was able to be determined. Circulatory changes associated with the physiological states of clamping and long-term (24hrs) emersion were investigated by sequentially injecting differently labelled microspheres under control and treatment states. The relative entrapment of the injected labels in the organs under the different physiological states reflected circulatory changes which these states produce. During emersion, increases in the relative perfusion of peripheral tissues which facilitate auxiliary gas exchange in other gastropods were not observed in H. iris. A decrease in relative perfusion of the foot suggested that this muscle may be isolated from the main circulation during emersion, and the simultaneous increase in microsphere entrapment observed in the gills but not other predominantly venous tissues, suggested arterial blood is, at least in part, bypassing the conventionally defined routes of flow during this physiological state. Clamping was associated with a decrease in relative perfusion of the foot muscle and more noticeably the adductor muscle, both of which appeared to be isolated from the rest of the circulation, and a simultaneous increase in microsphere entrapment in the gills strongly supports the existence of a hypothesised arterio-venous shunt. A protocol for measurement of cardiac output and stroke volume in H. iris using a modified reference sampling method was developed. Cardiac output and stroke volume (CSV) was estimated in resting animals and changes in the cardiac output associated with clamping were investigated Cardiac output was estimated at around 120ml kg-1 min-1, and CSV was around 1.2ml in the resting animals examined. A considerable decrease in cardiac output to around 40 ml kg-1 min-1 was observed when the animals were forced to clamp. This decrease was due to reductions in both heart rate and cardiac stroke volume. The method for using fluorescent microspheres to measure regional perfusion in H. iris was validated by comparison of control and treatment data and histological methods