The kinetics of zidovudine (AZT) distribution into rabbit cerebrospinal fluid (CSF) were studied during continuous infusion of AZT and after iv bolus administration. The CSF/plasma steady-state AZT concentration ratio was 0.192 ± 0.003. That this ratio is less than unity, and the clearance from the CSF due to bulk flow is much smaller than the total CSF-to-plasma clearance, suggests active CSF-to-plasma transport of AZT. Probenecid coadministration significantly enhances AZT distribution into CSF when plasma and CSF concentrations of AZT are at steady state during continuous infusion of AZT or at a transient steady state after a single iv bolus dose administration. A linear pharmacokinetic model which describes the distribution of AZT into CSF and relates intercompartmental clearances between CSF and plasma was developed and was used to analyze the results. This analysis showed that probenecid enhances the distribution of AZT into the CSF by its effect on clearances between plasma and CSF. The CSF exit-rate constant for AZT estimated during probenecid coadministration was significantly different from controls. Probenecid coadministration resulted in a 36% reduction in the CSF-to-plasma transfer-rate constant. Reduction in the CSF to plasma clearance of AZT is probably due to the effect of probenecid on the active CSF-to-plasma transport of AZT. The model analysis also indicates that probenecid may have increased the plasma to CSF clearance of AZT. There was an increasing trend in the steady-state CSF/plasma AZT concentration ratio with increasing plasma probenecid concentrations. These results are consistent with probenecid competitively inhibiting the CSF-to-plasma transport of AZT.