The use of oral gentamicin in infantile diarrhea is recommended by some authors. However, no data are available concerning the gastrointestinal absorption of gentamicin in infants when the mucosa of the small intestine is damaged. In this study, plasma gentamicin concentrations were measured in 14 infants suffering from prolonged diarrhea and treated with oral gentamicin (mean dose: 17 mg/kg every 8 hr). Plasma gentamicin levels were determined serially following the oral dose. Although marked individual and erratic temporal variations existed, average plasma gentamicin concentrations were low and stable (0.31 +/- 0.12 micrograms/ml). A positive correlation was found between the duration of the diarrhea and plasma gentamicin concentrations (r = 0.59, P less than 0.05). It is theorized that the damage to the mucosa as it occurs in prolonged diarrhea allows the absorption of the polar gentamicin molecule.