Experiments in animal models and cell culture systems have substantially improved our understanding of the complex pathophysiologic mechanisms of bacterial meningitis. Some studies have given new insights into the molecular mechanisms of the interactions of bacteria (Streptococcus pneumoniae, Escherichia coli) and cells making up the blood-brain barrier. There is increasing evidence that nitric oxide and excitatory amino acids are involved as mediators in the pathophysiology of bacterial meningitis, offering hope for new targets in adjunctive therapeutic strategies in this disease. Polymerase chain reaction on the cerebrospinal fluid is a rapid, specific method that has been shown to be useful for early diagnosis of tuberculous meningitis. Central nervous system toxoplasmosis is the most common cause of subacute focal encephalopathy in AIDS, which can be effectively treated with pyrimethamine-sulfadiazine.