1. 1. The effects of hypoxia, hyperoxia and hypercapnia on blood pH, pCO 2, pO 2, glucose, haematocrit and haemoglobin were investigated in the carp. Cyprinus carpio, with patent and ligated pneumatic ducts. 2. 2. Hypoxia and hypercapnia produced decreases in blood pH and pO 2 values and increases in blood pCO 2, glucose, haematocrit and haemoglobin values whereas hyperoxia increased only blood pO 2 and pCO 2 values in fish with patent pneumatic ducts. Some of these changes were more pronounced in ligated animals. 3. 3. Polyacrylamide gel electrophoresis of haemoglobin showed no significant changes in percentage composition during hypoxic conditions or during hyperoxia whereas hypercapnia caused definite changes in normal animals. Sham-operated and ligated fish showed a variety of changes for similar treatments. 4. 4. It is concluded that hypoxic conditions involve no conformational changes in carp haemoglobin and that variations in the organic phosphate level of the erythrocytes are probably responsible for the changes in oxygen affinity of carp haemoglobin. Abnormal conditions, on the other hand, cause pronounced effects possibly attributable to the involvement of different erythrocyte populations in the transport of various haemoglobin fractions.