Mangrove killifish, 'Rivulus marmoratus', were exposed to conditions encountered in their natural environment (i.e. air, freshwater to hypersaline waters, and elevated external ammonia). Fish were primarily ammonotelic when emersed, excreting 70-88% of total nitrogenous wastes (ammonia + urea) as ammonia. During air-exposure, nitrogen excretion was depressed, but ammonia remained the primary excretory product with ~45% excreted via NH3 volatilization probably across the cutaneous surface. Although urea cycle enzymes were detected, exposure to air resulted in only relatively small changes in enzyme activities. Under hypersaline conditions, urea excretion (JUrea) decreased and urea accumulated in body tissues, but these levels were ~60 times lower than expected if urea was contributing significantly to osmoregulation. Exposure to high external ammonia did not result in a shift towards ureotelism or a significant elevation of glutamate and glutamine. This study demonstrates that 'R. marmoratus' are able to continuously excrete ammonia in environments where ammonia excretion was unfavourable. Urea synthesis and excretion do not appear to be involved in the acclimation to the environmental conditions examined.