To study the behavior of antibiotics in the tissues, rats were sacrificed repeatedly in groups of six, after the injection of 25 mg/kg ampicillin, 100 mg/kg cephalothin or 10 mg/kg doxycycline. These antibiotics were bioassayed in ten different organs. Standards were established for each organ by using identical organs, thus avoiding errors caused by tissue binding or inhibition. Penetration into the tissue is very fast. Compared to serum levels, lung, muscle, heart, testicle and spleen, levels are higher for doycycline, lower for ampicillin and variable for cephalothin: for example, lung/serum ratio at 1 h is 2.2, 0.5 and 1.1, respectively; muscle/serum is 2.3, 0.2 and 0.18. The decrease in tissue levels parallels that in the serum for doxycycline, but is slower for ampicillin. The hepatic penetration of cephalothin is less than that of doxycycline or ampicillin. Levels are higher in the renal cortex than in the medulla for doxycycline, lower for cephalothin, and similar for ampicillin. The data enabled calculation of tissular pharmacokinetics. They have practical implications in the selection of antibiotics.