The effect of a diet high in protein (HP) (26%) or low protein (LP) (15%) content on kidney function and pharmacokinetic of nalidixic acid was studied in chickens. Nalidixic acid was given intravenously and orally in a dose of 25 mg/kg b. wt. with 15 days rest period between treatments. Nalidixic acid was determined by microbiological assay. The serum nalidixic acid concentrations were consistently higher in chickens that received a LP diet. The intravenous pharmacokinetics could be described by two compartments model with a t0.5(alpha) 0.13 +/- 0.008 hours and 0.17 +/- 0.008 hours; t0.5(beta) 3.07 +/- 0.07 hours and 2.56 +/- 0.05 hours; Cltot. 0.51 +/- 0.005 ml/kg/min and 0.59 +/- 0.01 ml/kg/min; Vdss 0.349 +/- 0.008 L/kg and 0.462 +/- 0.01 L/kg in LP and HP respectively. This suggests that the protein content of the diet modifies the distribution of body water and kidney function. Also, the results cleared higher recoveries of nalidixic acid in HP chickens.