After single oral or intravenous doses of doxycycline the concentrations found in the spinal fluid did not exceed 0.1 mcg/ml. Daily oral administration for 2-10 days gave a level in the spinal fluid of 0.1-0.76 mcg/ml. The mean value, 0.37 mcg/ml, corresponded to 14% of the concentrations found in serum. Further studies of the penetration of doxycycline in patients with a damaged blood-brain barrier should be performed before doxycycline can be recommended for the treatment of CNS-infections.