In a new canine experimental model the time concentration relationship of cephalothin in serum, urine, soft tissue interstitial fluid (STIF) and renal interstitial fluid (RIF) were compared simultaneously. Antibiotic concentration in RIF was less than urinary levels but exceeded the serum concentration. Urinary antibiotic concentration does not necessarily reflect concentration in the renal interstitium. This model helps to understand the basic pharmacokinetics of antibiotics in the renal interstitium where pyelonephritis occurs.