The effects of an anti-inflammatory drug, phenylbutazone, on the disposition of a commonly used cephalosporin, cefazolin, were studied in rabbits. The following investigations were made: mathematical analysis of blood levels obtained after i.v bolous injection of cefazolin, alone or combined with phenylbutazone (10 mg/kg), injection 4 hr before; protein binding by ultracentrifugation in vitro; and renal excretion and distribution in extravascular fluid obtained from s.c. tissue cages in vivo. Single i.m. injections of cefazolin (30 mg/kg) were administered either alone or in combination with phenylbutazone (10 or 100 mg/kg i.m.) or 2 or 4 hr before. The mathematical analysis disclosed a competition of phenylbutazone on protein binding of cefazolin. In vitro, phenylbutazone reduced the extent of protein binding of the antibiotic (74-80 to 47-59%). Cefazolin appeared at higher concentrations in extravascular fluid in the presence of phenylbutazone than when administered alone. Phenylbutazone appeared to be responsible for a dose-dependent effect on renal excretion of cefazolin i.e., a reduction of secretion at low doses (10 mg/kg) and a possible reduction of tubular reabsorption at high doses (100 mg/kg). A bidirectional transport of cefazolin in rabbit tubules was thus shown. The interaction of phenylbutazone on the on the disposition of cefazolin appeared also dependent on the time of injection of the former and on the mode of administration of the antibiotic.