The acute toxicity of endosulfan in juvenile rainbow trout (Oncorhynchus mykiss, 10.61 +/- 1.69 g) was evaluated in glass aquaria under static conditions. Nominal concentrations of endosulfan in the toxicity test ranged from 1.3 mu g l(-1) to 29 mu g l(-1). The concentrations of endosulfan that killed 50% of the rainbow trout within 24-h (24-h LC50), 48-h LC50, 72-h LC50, and 96-h LC50 were 19.78, 8.89, 5.28, and 1.75 mu g l(-1), respectively. None of the unexposed control fish died, and the first fish died 4 h after exposure to 26.3 mu g l(-1) of endosulfan. Survival of fish was significantly increased with increasing fish size and decreased with decreased fish size at the same temperature (p < 0.001). Temperature also had a significant effect on survival of fish. Alkalinity at levels above 20 mg l(-1) as CaCO3 significantly increased survival of fish at 19.78 mu g l(-1) of endosulfan. Increasing alkalinity from 20 mg l(-1) as CaCO3 to 42 or higher concentrations tested in this study (121 mg L-1 as CaCO3) significantly increased survival of fish (p < 0.01). Total hardness ranging from 55 mg l(-1) as CaCO3 to 126 mg l(-1) as CaCO3 did not affect survival of fish exposed to endosulfan. Endosulfan toxicity was found to be irreversible when fish were exposed to minimum concentrations of endosulfan tested. Histologically, fish gills had lamellar edema, separation of epithelium from lamellae, lamellar fusion, and swelling of the epithelial cells. Melanomacrophage centers were scattered throughout the trunk kidney, head kidney, and spleen. The liver of endosulfan-exposed fish had severe focal necrosis. None of these lesions were seen in unexposed control fish. Results indicate that alkalinity, temperature, and fish size affect endosulfan toxicity of rainbow trout. (c) 2005 Elsevier Ltd. All rights reserved.