Radial oxygen loss (ROL) was measured as a function of distance behind the tip for roots of the seagrass Halophila ovalis (R.Br.) Hook f. The effects of shoot illumination and leaf area on ROL were also examined as were the porosity and anatomy of the roots, rhizomes and petioles of H. ovalis. For plants taken from the Swan Canning Estuary, Western Australia, the porosities of roots, rhizomes and petioles were 15%, 27% and 17%, respectively. ROL from roots in an O-2-free and saline medium was measured using root-sleeving cylindrical platinum O-2 electrodes. The shoots were submerged in aerated seawater during the measurements. ROL was substantially higher when the shoots were exposed to saturating light and it decreased markedly in the dark. These findings, and experiments in which the leaves were excised, show that O-2 lost radially from the roots was photosynthetically derived. Moreover, ROL showed a marked gradient along the root; ROL decreased from an average maximum value of 72 ng cm(-2) min(-1) at 0.5 cm behind the root tip to only 4 ng cm(-1) min(-1) at 3 cm, the most basal position tested. These data show that roots of H. ovalis contain 'a barrier to ROL' in the more basal regions, an adaptation shown by other workers to enhance the growth of roots of wetland macrophytes into anaerobic sediments. (C) 1999 Elsevier Science B.V, All rights reserved.