Thirteen male subjects received 1 mg of either gentamicin or netilmicin per kg, first intramuscularly and then intravenously. After the intramuscular dose, concentrations of gentamicin in the serum were more variable than those of netilmicin. After the intravenous dose, the distribution phase of netilmicin was twice as rapid as gentamicin. The average half-times of the elimination phase were similar, but there was marked variability among the subjects receiving gentamicin. Serum clearance of netilmicin was more rapid than that of gentamicin and could not be attributed to renal elimination. The data indicate that, after intramuscular administration, netilmicin may produce more predictable blood levels than gentamicin and suggest that the body distribution of netilmicin may differ from that of gentamicin.