Tobramycin, sisomicin, and amikacin were compared with gentamicin in the therapy of experimental E. coli meningitis in rabbits. Meningitis was produced in 40 animals by intracisternal injection of 10(5) E. coli. Three dosages of each antibiotic were administered intravenously over 8 hours. Serum and CSF samples were obtained at 0,2,4,6, and 8 hours for determination of aminoglycoside concentrations and CSF bacteria counts. The four aminoglycosides demonstrated comparable penetration into the CSF. The mean percent penetration (CSF conc./serum conc. X 100%) with the three dosages was 10 to 50% for gentamicin, 8 to 23% for tobramycin, 6 to 16% for sisomicin, and 11 to 23% for amikacin. This variation in penetration reflected individual differences in each dosage group and the increase in percent penetration that was observed during therapy. Sisomicin and gentamicin were consistently bactericidal in vivo. Mean CSF bacterial titers gentamicin were consistently bactericidal in vivo. Mean CSF bacterial titers decreased 3.07 logs in rabbits treated with sisomicin and 2.44 logs in animals treated with gentamicin. Even through CSF concentrations were comparable, the group treated with tobramycin demonstrated only a 0.64 log decrease and the amikacin group had a 0.45 log increase in mean CSF titers. The bactericidal effect of sisomicin appeared to be more rapid than that of gentamicin. During the first 2 hours of therapy CSF titers declined 1.02 logs in animals treated with sisomicin as compared to 0.37 log in animals receiving gentamicin even though gentamicin concentrations were higher (mean 7.4 vs. 4.1 gm./ml.). Sisomicin also demonstrated greater bactericidal activity than the other aminoglycosides in normal CSF in vitro. The results of this study suggest that sisomicin may be of value in the therapy of E. coli meningitis.