The penetration of aztreonam into cerebrospinal fluid was 7 to 15% and 9 to 25%, respectively, in experimental Haemophilus influenzae type b and Escherichia coli K1 meningitis. Aztreonam was effective in reducing the number of organisms in cerebrospinal fluid after single-dose and continuous infusion administration, and the median bactericidal titers in cerebrospinal fluid were 1:32 against both meningeal pathogens.