The concentrations of Gentamicin, Ampicillin and Cephaloridine in thigh rat bone were studied. Measurements were done after a 24 h administration for 3 different times. Measurable quantities in the bone for the three antibiotics were detected. The best levels in relation with the M.I.C. of 4 microorganisms occurred in Cephaloridine (and in measurements of serum and bone (microgram/g). The concentration in the serum does not represent concentration in the bone. The most convenient parity was for Cephaloridine. The elimination of the drug in the bone was impressive after 2h. The slower fall occurred in Cephaloridine. Ampicillin had the same level in 4 and 6h. Gentamicin and Cephaloridine had failed to present measurable quantities in some of the animals of the last measurement.