The air-breathing Singhi catfish Heteropneustes fossilis was kept inside moist peat for 1 month mimicking their normal habitat in summer and the role of ureogenesis for their survival in a water-restricted condition was studied. The ammonia excretion rate by the mud-dwelled fish increased transiently between 6 and 12 h of re-immersion in water to approximately between eight and 10-fold, followed by a sharp decrease almost to the normal level at the later part of re-immersion. The urea-N excretion by the mud-dwelled fish increased to approximately 11-fold within 0-3 h of re-immersion, followed by a gradual decrease from 9 h onwards. The rate of urea-N excretion by the mud-dwelled fish, however, remained significantly higher (approx. threefold more) than the control fish even after 36-48 h of re-immersion. Although there was a significant increase of both ammonia and urea levels in the plasma and other tissues (except ammonia in the brain), the level of accumulation of urea was higher than ammonia in the mud-dwelled fish as indicated by the decrease in the ratio of ammonia: urea level in different tissues including the plasma. The activities (units/g tissue and /mg protein) of glutamine synthetase and three enzymes of the urea cycle, carbamyl phosphate synthetase, argininosuccinate synthetase and argininosuccinate lyase increased significantly in most of the tissues (except the brain) of the mud-dwelled fish as compared to the control fish. Higher accumulation of ammonia in vivo in the mud-dwelled Singhi catfish is suggested to be one of the major factors contributing to stimulation of ureogenesis. Due to this physiological adaptive strategy of ureogenesis, possibly along with other physiological adaptation(s), this air-breathing amphibious Singhi catfish is able to survive inside the moist peat for months in a water-restricted condition.