Various cytological, physiological and biochemical properties of the blood have been examined in different life cycle stages of the lamprey Geotria australia. Initial experiments to determine physiologically realistic upper temperatures yielded an ultimate lethal temperature for larvae of 28.3oC. Since this value is comparatively low, it helps explain the restriction of G. australis to more southern rivers in Australia. The haematocrit (46.2%), red blood cell number (1.231 X 106 cells mm-3) and haemoglobin concentration (11.8g 100ml-l) of adult G. australis are more typical of comparable stages in holarctic lampreys than those of their ammocoetes (41.5%, 1.809 106 cells mm-3, 11.1 g 100 ml-1). During metamorphosis, the pattern of change in haemopoietic sites, haemoglobin electropherograms and the proportion of mature erythrocytes, indicate that erythrocytes containing larval and adult haemoglobins always originate in different structures. The molecular weights (c 17000) and pI values (5.1-6.4) of G. australis haemoglobins are similar to those of other lampreys. The P50 of larval blood is very low, while that of adult blood is more comparable to that of other lampreys (cf. 0.92 mm Hg for ammocoete and 10.3 mm Hg for adult at pH 7.75 and 15oC). Increases in temperature do not affect the Bohr shift (range -0.16 to -0.27) but are accompanied by a shift of the oxygen dissociation curve to the right. The major plasma iron binding proteins have molecular weights of 354,000 in the ammocoete and 296,000 in the adult and contain 20 and 4 subunits respectively. The larval IBP is thus ferritin-like while that of the adult is transferring-like, features consistent with their respective pI values, Fe/protein ratios and ultrastructure. Total plasma iron was 19,760 ug 100 ml-1 in larvae and 34 ug 100 ml-1 in adults. Iron granules were present in the columnar cells of the posterior intestine in small or negligible amounts in the Petromyzonidae and in very large amounts in the Mordaciidea. While some iron was found in the same location in the Geotriidae, it was also present in very large concentrations elsewhere in the body. The high haemoglobin concentration and blood iron levels, and the large iron deposits and the low P50 in larval G. australis, probably represent adaptations to reduced environmental oxygen tensions.