Usability of a repeated renal biopsy was examined for the estimation of drug residue in the pig kidneys. The renal biopsy was carried out under laparoscopic observation. Eleven castrated pigs were given intramuscularly 1 mg (potency) of gentamicin (GM) per kg body weight. The renal parenchyma was sampled by the biopsy 3, 10, 20, 30, 40 and 50 days after administration. Two pigs were sacrificed to obtain the kidneys immediately after each biopsy. No clinical manifestations were observed except transient hematuria in all biopsied pigs. The mean concentration of GM in the biopsied samples decreased similarly to that in the kidneys of sacrificed pigs. There was no significant difference in the analytical values between the biopsied samples and those obtained by sacrifice immediately after the biopsies. These results suggest that the estimation of drug residue in the pig kidneys by the repeated biopsy is useful to determine the withdrawal time of drugs remaining predominantly in the kidneys.