Diurnal and long-term excretion by leaves of Avicennia marina seedlings growing in aqueous culture was correlated with substrate salinity and transpiration. Excretion was greater in 100% than 50% seawater but the reverse was true for transpiration. The diurnal excretion pattern, with exudation minimal during the day and maximal during the night, showed a negative correlation with the daily transpiration pattern. The total amount of salt excreted, however, showed a positive correlation with the total amount of water transpired. Root and xylem sap salinities were linearly related to substrate salinity but leaf Na+ increased to a maximum, indicating that control of leaf salt content is at the foliar, rather than the root level.