Renal function was evaluated in adult rainbow trout (Salmo gairdneri) dosed i.a. with rotenone at 225 and 275 μg/kg. The chemical composition of urine samples and urine flow rates collected over a 5-h pretreatment period were compared with hourly urine samples collected over a 5-h posttreatment period. Significant increases in osmolality and in concentrations of sodium, potassium, chloride, glucose, and total protein were observed in the urine of treated fish. Urine solute concentrations reached maximum values within 1 to 3 h after treatment and decreased thereafter, indicating that the effects were reversible. Concentrations of sodium and chloride were highly correlated in 2-h posttreatment urine samples at the low (r = 0.922) and high (r = 0.981) rotenone treatments. Urine flow rates were reduced in trout at each dose of rotenone but the decrease in volume of urine voided was not dose-dependent. In a separate study, [¹⁴C]polyethylene glycol was used as a filtration marker to determine the effect of rotenone treatment (225 &mu:g/kg) on urine flow rate, glomerular filtration rate, and renal water reabsorption. We showed that posttreatment urine flow rates were reduced partly by reduced glomerular filtration and partly by increased water reabsorption. Transient increases in plasma osmolality and hematocrit also were observed 0.5 h after rotenone treatment.