The present investigation was undertaken to study the effect of an abrupt change in the salinity of the medium on the oxygen consumption and ammonia-N excretion of the marine penaeid shrimp Metapenaeus monoceros (Fabricius). Results showed that in both low-saline (5‰) as well as high-saline (35‰) acclimated shrimps the respiratory rates were significantly lower in midrange salinities (20‰ and 25‰) and significantly higher in both low (5‰, 10‰, and 15‰) and high (30‰ and 35‰) salinities. A significant increase in ammonia-N excretion was observed when high-saline acclimated shrimps were abruptly exposed to different grades of low-saline media. Exposure to different grades of high-saline media on the other hand induced a significant decrease in ammonia excretion rate of low-saline acclimated shrimps. The O:N ratio (ratio of oxygen consumed to nitrogen excreted in atomic equivalents) showed a decreasing trend when the high-saline acclimated shrimps were abruptly exposed to low-saline media, indicating a shift towards protein dominated metabolism. A reverse trend could be observed in the O:N ratio when the shrimps were exposed to high-saline media indicating a shift towards lipid dominated metabolism in high salinities. Thus, there appears to be a shift in energy substrate utilization in these shrimps from protein dominated metabolism in low salinities (5‰, 10‰, 15‰ and 20‰) to lipid/carbohydrate dominated metabolism in high salinities (25‰, 30‰ and 35‰).