Vosláfiová E., V. Pi‰tûková, Z. Svobodová: Nitrite Toxicity to Danio Rerio: Effects of Fish Age and Chloride Concentrations. Acta Vet. Brno 2006, 75: 107-113.The aim of the present study was to compare the acute nitrite toxicity for Danio rerio at the age of 20 -25 days (larval stage) and 2 -3 months (juvenile stage) at various chloride concentrations. The semistatic method according to OECD 203 was used in the tests. Nitrite (NO 2 -) is a naturally occurring anion in fresh and saline waters, its concentration in oxygenated waters is typically less than 0.005 mg·l -1 (Lewis and Morris 1986). Elevated concentration of nitrite can be found in water receiving nitrogenous effluents, in various hypoxic environments or in effluents from industries producing metal, dyes, and celluloid (Pitter 1999). The increased nitrite concentrations in water also present one of the frequent problems encountered both in aquaria and on fish farms. Sources of nitrites are mainly products of fish metabolism and decaying remains of food and faeces. In aquaria, fish are most frequently poisoned by nitrites that accumulate in the tank as a result of insufficient efficiency of biological filters (Adamsson et al. 1998;Etscheidt 2003; Dvofiák 2004;Svobodová et al. 2005a). An elevated ambient nitrite concentration is a potential problem for freshwater fish, since nitrite is actively taken up across the gills in competition with chloride (Maetz 1971;Williams and Eddy 1986;Jensen 2003).Nitrite toxicity to fish varies considerably and depends on a large number of external and internal factors whose importance is continuously verified and re-assessed. One of the important factors influencing nitrite toxicity for fish is the fish size and age. Lewis and Morris (1986) stated that small fish, even larvae, are unlikely to be more sensitive to nitrite than larger fish of the same species, and that there is definite evidence for some species that very small fish are less vulnerable to toxicity than fish of intermediate or large size. A study by Perrone and Meade (1977) indicated that coho salmon (Oncorhynchus kisutch) fry have greater tolerance to nitrite than coho yearlings. This phenomenon has been also observed in other species of salmonids. An early study by Smith and W illiams (1974) on rainbow trout (Salmo gairdneri) showed that small fish were less sensitive to 24h exposures of nitrite than larger ones. Russo et al. (1974) showed that rainbow trout larvae were slightly less susceptible to nitrite than larger fish and subsequent studies by Russo and ACTA VET. BRNO 2006, 75: 107-113