The present work deals with the in vitro and in vivo distribution of ofloxacin and ciprofloxacin in erythrocytes. In vitro studies were carried out in standard solutions prepared using fresh blood for a concentration range between 100 and 0.25 micrograms mL-1. A 5 mg kg-1 bolus dose was administered to rabbits and erythrocyte and plasma kinetics were determined over 8 h. A linear model was used to establish the relationship between plasma and erythrocyte concentrations of both quinolones in vitro. The mean partition coefficient values obtained were 1.04 +/- 0.02 and 1.32 +/- 0.03 for ofloxacin and ciprofloxacin, respectively. A decrease in the ciprofloxacin partition coefficient was observed at higher concentrations. Values ranged between 2.54 +/- 0.40 and 1.38 +/- 0.15 as the concentrations increased. The partition coefficients obtained from the linear relationship between plasma and erythrocyte concentrations established from the in vivo data were 0.80 +/- 0.58 for ofloxacin and 0.61 +/- 0.30 for ciprofloxacin. In vivo plasma and erythrocyte data analysis was performed by a deconvolution method and the theoretical transfer curves in erythrocytes were estimated. The distribution of both quinolones to erythrocytes is very rapid, probably due to a high permeability of erythrocyte membranes to these drugs. This was also confirmed by the parallelism between plasma and erythrocyte kinetics.