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Digoxin and gentamicin are widely used in pediatric medicine, and therapeutic monitoring is mandatory because of their narrow margin of safety and wide interpatient and intrapatient pharmacokinetic variabilities. Saliva sampling may be of potential interest, especially in children, in whom blood sampling is often difficult. In 11 children treated with digoxin for various cardiac conditions, and in 24 children treated with gentamicin (14 patients were administered gentamicin three times a day, and 10 once-daily), drugs levels were measured in plasma and saliva. There was no correlation between plasma total or free digoxin concentrations and saliva levels, precluding the clinical use of the saliva test for digoxin. No correlation was found between plasma gentamicin concentrations and saliva levels when the drug was administered three times a day; however, good correlation was found when the drug was administered once-daily (r2 = 0.89, p < 0.0001). Saliva may be used as a noninvasive method of measuring gentamicin serum concentrations to guide dosage adjustments in patients administered the drug once-daily.