A study was made of the pharmacokinetic behaviour in plasma and urine of cefazolin in seven healthy volunteers following parenteral administration of sodium cefazolin (1250 mg) and a sustained release formulation containing sodium cefazolin: cefazolin-dibenzylamine (1:4) at a total dose of 1250 mg, both formulations being administered over 1 week by the i.m. route. Cefazolin concentrations in plasma and urine were determined by a HPLC technique. Kinetic analysis of the experimental results was performed using an open single-compartment kinetic model and a sustained release model for the drug administered as standard formulation and the sustained release formulation, respectively. The results obtained point to significant variations in the pharmacokinetic profile of the drug when administered in the DDS. The time of cefazolin at levels greater than 1 microgram ml-1 was 16.03 +/- 2.51 and 47.76 +/- 14.18 h-1 after administration of the standard formulation and the DDS, respectively. The urinary excretion rate curves also show the existence of sustained drug levels in the urine following administration of the DDS. The renal clearance of the drug did not show statistically significant differences between the two formulations administered. The process of release of cefazolin from the cefazolin-dibenzylamine complex proved to be a first order kinetic process. The release constant of the antibiotic was calculated according to three different methods: the Wagner-Nelson method; the statistical moments method; and the fitting of the plasma levels curves to a compartmental model considering release and absorption. The values obtained for this constant ranges from 0.026 +/- 0.02 h-1 calculated with the method of statistical moments to 0.094 +/- 0.03 h-1 as calculated by the equation derived for the plasma fitting of cefazolin administered as DDS.