The effect of probenecid on the pharmacokinetics of ceftizoxime was studied. Twelve healthy male volunteers first received 1 g of ceftizoxime either by iv rapid injection or by im injection. One week later, each subject was again given a similar dose of ceftizoxime, one hour after the ingestion of 1 g of probenecid. Serum and urinary concentrations were determined by a HPLC method. Ceftizoxime half-life was increased by probenecid from 1.7 to 2.3 h in the iv group, and 1.9 to 2.8 h in the im group (P less than 0.05, paired t-test). Probenecid increased the area under the serum concentration-time curve by 49% in both groups. There was no significant change in the volume of distribution following probenecid. This study confirms that ceftizoxime is eliminated by glomerular filtration and tubular secretion and that the latter is inhibited by probenecid.