The effects of acidic water on Saratherodon niloticus were in vestigated by measurement of ionic concentrations, blood gas levels and other commpnents in blood samples drawn from the dorsal aorta and coudal vein at intervals throughout a 6-day exposure period to water of pH 4.0 and pH 5.0, maintained by addition of sulphuric acid, and non-adjusted control water of pH 6.80+0.01. The 6-day-mortality of fish in pH 4.0 was 100% while no mortality occurred among fish in pH 5.0 or control water. After 2 days of immersion in water of pH 4.0., plasma pH and HCO(,3)Na+ and Cl-showed progressive decreases, while haematocrit indes, haemoglobin content and plasma K+, glucose and protein increased, and after 4 days arterial PO(,2) levels, declined. After 6 days pH and plasma Na+ of fish in water of pH 4.0 were 5.92 and 72.30 mmoldm(-3) while those of fish in control pH were 7.04 and 150.75 mmoldm(-3). Fish in water of pH 5.0 and control water showed little changes in plasma ionic concentration and blood gas level. Estimations of the amount of gill mucus by examination of the site and intensity of fluorescence of acridine orange stained gill sections, revealed a marked increase with increasing exposure time to water of pH 4.0 and a slight increase with time of exposure to pH 5.0 while little change occurred during exposure to control pH water. Cardiographic recording of fish immersed in water of pH 3.0 showed no significant deviations from normal as long as respiratory cycles were maintained, and no decrease in the capacity to respond to hypoxic water by increased respiratory-cardiac frequency ratio, was shown by fish immersed in water pH 3.0 These data indicate an ability to regulate plasma pH and blood ionic concentration when exposed to water of pH 5.0 Fish exposed to water of pH 4.0, however, are incapable of complete regulation resulting in acidaemia, hyponatraemia, subsequence respiratory depression and hypoxaemia, and eventual death. The mechanisms underlying the response to acid water and the changes leading to death are considered.