The elimination of zidovudine (AZT) from cerebrospinal fluid (CSF), its distribution from CSF to brain tissue, and its transport from brain extracellular fluid (ECF) to plasma were studied during intracerebroventricular (i.c.v.) infusion in unanesthetized rabbits. The effect of probenecid (PBD) on these transport processes was also studied. The concentration of AZT in brain ECF was measured by microdialysis with retrodialysis calibration for in vivo recovery. Plasma and CSF were sampled and analyzed for AZT and PBD using HPLC. The elimination of AZT from CSF showed nonlinear characteristics as the i.c.v. infusion rate was increased to 1 mg/h kg. The estimated maximum transport capacity and dissociation constant were 3.5 micrograms/min kg and 127 micrograms/mL, respectively. The total linear elimination clearance from CSF was 0.0073 mL/min kg. The spatial distribution of AZT in brain during i.c.v. infusion was simulated using a mathematical model which describes diffusive solute transport in brain ECF and efflux across the blood-brain barrier. This analysis yielded a brain to plasma efflux rate constant of 0.040/min. This parameter and the elimination clearance from CSF decreased significantly by the end of an 8-hour period during which PBD was infused intravenously at a rate of 15 mg/h kg.