A study was made of the effects of antibiotics and corticosteroids on parameters that reflect brain dysfunction and potential neurological damage in experimental pneumococcal meningitis in rabbits. Brain water content was 398 ± 10 g/100 g dry weight in normal rabbits and 410 ± 11 g in rabbits after 24 hr of infection (P <.001). Cerebrospinal fluid (CSF) lactate levels increased from 16.3 ± 3.4 mg/dl to 69.5 ± 28.2 mg/dl (P < .001), and CSF pressure increased by +8.3 ± 3.6 mm Hg (P < .005) over the same interval. Antibiotic therapy with ampicillin sterilized CSF and normalized CSF pressure and brain water content in all animals within 24 hr, while CSF lactate levels remained elevated. Administration of methyl prednisolone, 30 mg/kg, or dexamethasone, 1 mg/kg, 15and 22 hr after infection completely reversed the development of brain edema, but only dexamethasone also significantly reduced the increase in CSF lactate level (43.8 ± 12.3 mg/dl) and CSF pressure (+ 1.8 ± 2.7 mm Hg). Methyl prednisolone did not significantly affect pressure or lactate levels.