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The optimal regimen of benzathine penicillin G for secondary prevention of rheumatic fever is controversial. Data from serum pharmacokinetic studies do not fully agree on the period of protection after drug administration. Data from concentration of penicillin in tonsils may provide additional information.To evaluate penicillin concentrations in palatine tonsils and in sera 1, 10, 14 and 21 days after intramuscular injection of benzathine penicillin G 40 000 IU/kg, 58 children between 4 and 12 years of age with chronic tonsillitis and indication for tonsillectomy were given the study drug 1, 10, 14 or 21 days before surgery. Blood and tonsil samples were obtained during surgery, and penicillin concentrations were determined microbiologically by the agar well diffusion technique.Mean serum penicillin concentrations 1, 10, 14 and 21 days after drug administration were, respectively, 0.080, 0.031, 0.023 and 0.014 μg/ml. Mean penicillin concentrations in tonsils at 1, 10, 14 and 21 days were 0.023, 0.010, 0.007 and 0.002 μg/g, respectively. Detectable penicillin concentration in tonsils (method sensitivity, 0.006 μg/g) was obtained in all patients on Day 1 and in 91% and 83.3% of patients on Days 10 and 14, respectively. By Day 21 this proportion was reduced to 30%.The results of this study suggest that penicillin values may be inadequate for prevention of rheumatic fever by Week 3 of administration in a majority of children.