The renal effects of gentamicin and cephaloridine were investigated by determining the changes in renal enzyme excretion. Both drugs provoked significant elevations or urinary AP-, LAP-, or LDH-activities. This result permits the conclusion that both drugs take influence on the kidney. The exact pathomechanism leading to this increased renal enzyme excretion was not elucidated. The fact that gentamicin in the applied dosage (about 30% of LD50) provoked more pronounced changes than cephaloridine (about 20% of LD50) could be explained by an in-vitro interference of cephaloridine with the enzymatic activities investigated in this study. In the discussion, the changes elicited in the same model by other antibacterial compounds were compared with the data obtained with gentamicin and cephaloridine. In closing, implications, restrictions, and validity of renal enzyme excretion studies in rats for screening nephrotoxic properties of a drug for human use are briefly discussed.