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Randall, D. J., J. M. Wilson, K. W. Peng, T. W. K. Kok, S. S. L. Kuah, S. F. Chew, T. J. Lam, and Y. K. Ip. The mudskipper, Periophthalmodon schlosseri, actively transports [MATHEMATICAL EXPRESSION NOT REPRODUCIBLE IN ASCII] against a concentration gradient. Am. J. Physiol. 277 (Regulatory Integrative Comp. Physiol. 46): R1562-R1567, 1999.--Periophthalmodon schlosseri can maintain ammonia excretion rates and low levels of ammonia in its tissues when exposed to 8 and 30 mM [NH.sub.4]Cl, but tissue ammonia levels rise when the fish is exposed to 100 mM [NH.sub.4]Cl in 50% seawater. Because the transepithelial potential is not high enough to maintain the [MATHEMATICAL EXPRESSION NOT REPRODUCIBLE IN ASCII] concentration gradient between blood and water, ammonia excretion under such a condition would appear to be active. Branchial [Na.sup.+]-[K.sup.+]-ATPase activity is very high and can be activated by physiological levels of [MATHEMATICAL EXPRESSION NOT REPRODUCIBLE IN ASCII] instead of [K.sup.+]. Ammonia excretion by the fish against a concentration gradient is inhibited by the addition of ouabain and amiloride to the external medium. It is concluded that [Na.sup.+]-[K.sup.+]-ATPase and an [Na.sup.+]/[H.sup.+] exchanger may be involved in the active excretion of ammonia across the gills. This unique ability of P. schlosseri to actively excrete ammonia is related to the special structure of its gills and allows the fish to continue to excrete ammonia while air exposed or in its burrow. ammonia; [MATHEMATICAL EXPRESSION NOT REPRODUCIBLE IN ASCII]; ouabain; amiloride