The pharmacokinetics of 5-fluorouracil (5-FU) was investigated in tumor-free anesthetized rats. Blood samples in the carotid artery and in the right atrium were sampled by cannulation during the steady-state infusion of 5-FU. Serum and blood samples were assayed by HPLC. After comparing the steady-state concentration of 5-FU in the right carotid artery with that in the right atrium, it was concluded that there was no pulmonary elimination of 5-FU in the anesthetized rat.