The relationships of antibiotic concentrations in interstitial fluid and serum antibiotic concentrations were compared after intravenous injection of cephalothin sodium in three studies in dogs. Antibiotic concentrations in the serum and interstitial fluids showed the same time-concentration response regardless of dose given. The immediate high serum concentrations decreased to less than that of the interstitial fluid concentrations by two hours. After fifteen minutes, the interstitial fluid concentration was not significantly different from maximum levels recorded later. It was found that diffusion of cephalothin into dissected tissues occurs, though the diffusion was delayed when compared to that in uninjured tissues. The concentrations were the same between one and two hours.