The scale-less carp (Gymnocypris przewalskii) inhabits Lake Qinghai located on the Qinghai-Tibet plateau (elevation, 3200 m) in western China. The lake waters are alkaline (pH 9.4, titratable alkalinity=30 mmol l(-1)), Mg(2+)-rich (18.7 mmol l(-1)), Ca(2+)-poor (0.30 mmol l(-1)) and saline (9 per thousand ). These fish make annual spawning migrations into freshwater rivers. We investigated the physiology of nitrogen excretion and ionoregulation of fish from the lake and river. Fish from both waters were ammonotelic, although ammonia-N excretion rates were lower in lake fish (175 vs. 344 micromol kg(-1) h(-1), P<0.05) resulting in unusually high levels of ammonia in blood plasma (2.23 vs. 0.32 mmol l(-1)), bile, liver, muscle and brain. Exposure to 0.4 mmol l(-1) total ammonia in lake water ([NH(3)]=0.16 mmol l(-1)) killed fish within 8 h. River fish survived exposure to 1.0 mmol l(-1) total ammonia in river water at pH 8.0 ([NH(3)]=0.023 mmol l(-1)) for 24 h suggesting high ammonia tolerance in lake fish. High glutamate dehydrogenase and glutamine synthetase activities in tissues probably allow the fish to alleviate ammonia toxicity by amino acid accumulation. Neither lake nor river fish relied on urea excretion to remove excess N. Urea-N excretion rates were below 20 micromol kg(-1) h(-1) for both groups, and levels of urea in plasma and tissues were moderate. When exposed to elevated ammonia, urea-N excretion increased slightly (approximately 50 micromol kg(-1) h(-1)) and liver and muscle urea levels increased in the river fish. Plasma ion levels were within the range typical of cyprinids, but river fish had significantly higher plasma [Na(+)] and [Cl(-)] and lower [K(+)] than fish from the lake. During 48-h lake-to-river water transfer, plasma Na(+) and Cl(-) levels rose significantly. Significantly higher Na(+)/K(+)-ATPase activity in the gills of river fish may be related to the higher plasma ion levels. Plasma [Mg(2+)] and [Ca(2+)] were tightly regulated despite the great differences in the lake and river water levels.