This study investigated the effects of low pH (6.0-3.6) on the respiratory metabolism and blood acid-base balance in grass carp, Ctenopharyngodon idellus, using techniques of modern transduction and in vivo blood sampling with arterial cannulae. The general model of effects of low pH on respiration by juvenile grass carp was an increase in ventilation rate, depth of breathing and cough responses. Oxygen consumption rose initially and decreased subsequently. Ventilation rate, depth of breathing and Oxygen consumption were not affected at pH 5.6. At pH 4.6, ventilation rate and cough rate increased apparently, but depth of breathing and Oxygen consumption rate were uneffected. Exposure to pH 3.6 resulted in highly significant rises in ventilation rate, cough rate and depth of breathing, and decrease in Oxygen consumption. Also observed was the secretion of large quantities of mucus on the gills. At pH 6.0, adult grass carp showed a slight fluctuation in blood pH and (HCO_3~-) but there was no response in blood PO_2 and PCO_2. At pH 5.0, blood pH and (HCO_3~-) decreased significantly, but seemed to attain a new balance at a lower level, PaO_2 and PaCO_2 remained uneffected. At pH 4.0, the acid-base status of the blood was severely affected, there being a drastic loss in blood (HCO_3~-) and a significant decrease in blood pH. Blood PCO_2 increased and blood PO_2 decreased significantly. This study showed that fish possess very powerful self-regulation ability in order to maintain normal gas metablism. Respiratory functions of grass carp could recover after prolonged exposure to the sublethal level low pH (5.6). At lethal levels (pH 3.6-4.6), gas metabolism of grass carp was severely disturbed and blood PO_2 decreased. In the end. Oxygen deficiency in tissues of organisms occurred. Anoxia is one of the important mechanisms which resulted in death of grass carp caused by acid water. Exposure to low pH resulted in acidaemia in grass carp. This phenomenon aggravated physiological damages in the fish. Loss of blood (HCO_3) played an important role in the occurrence of acidaemia of grass carp. The major mechanism which causes acidaemiain grass carp may be a H~+ influxe to the gill. Grass carp is an acid-sensitive fish compared with Salmonids. The response of blood acid-base status in grass carp to acidification to pH 6.0 suggested that water pH must be keep to 6.0 or higher to ensure the health of the grass carp and avoid damages caused by acid water. The sensitivies to low pH of the responses of the grass carp were in the order: cough response > ventilation rate > Oxygen consumption rate > depth of breathing. From the practical point of view, cough rate and ventilation rate can be used as indicators in biological monitoring and early warning system because they are easy to measure and highly sensitive.