The present study was undertaken to compare the ability of three cephalosporins--cephalothin, cephaloridine and cefuroxime--and a cephamycin--cefoxitin--to penetrate into extravasal compartments. The study was performed using female adult rabbits with subcutaneously implanted steel net cages. All antibiotics were given in a dose of 50 mg/kg b.w. parenterally as i.v. injections. The concentrations of the different antibiotics were determined simultaneously in serum and in fluid obtained from the cages. The concentrations were followed after single doses as well as after repeated doses using an agar well technique for the assays. To control the experimental conditions electrolyte and protein contents of the fluid in the different cages were followed and were found to be stable during the experimental period. All antibiotics were rapidly eliminated from serum. The concentrations in the cages reached its maximum 1.5-3 hours after administration. The highest concentrations in the extravasal compartments were obtained with cephaloridine and the lowest with cephalothin with the values of the other antibiotics ranging in between.