Fluid therapy is the most important component involved in the stabilization of postrenal azotemia, however there is not yet a consensus about the ideal rate of fluid therapy after relief of urethral obstruction. Fifteen adult cats were divided in two groups: High Rate Group (HRG) and Low Rate Group (LRG). At predetermined time points was assessed venous blood pH, pCO2, pO2, bicarbonate concentration, BE, creatinine, BUN, Na+, K+ and urinary output. HRG had a lower glomerular filtration rate, characterized by a lower urinary output and a slower stabilization of BUN, creatinine, acidbase balance and potassium. The infusion of high rates of fluids in cats after a urethral obstruction must be undertaken with caution since it can lead to death due to the occurrence of pleural effusion and the absence of appropriate restoration of electrolyte balance and glomerular filtration.