Fifty-four healthy guinea pigs were studied histologically for evaluation of Tobramycin sulfate ototoxicity. They were treated with 50, 100, 150 and 200 mg/kg/day under different schedules. Loss of hair cells occurred when dosage of 1,200 mg/kg was reached. Animals receiving 700 mg/kg were without damage to the inner ear. Outer hair cell loss occurred first in the basal turn, spreading upward. Inner hair cell loss began in the apical turn, progressing downward. When a total dose of 1,400 mg/kg was reached by (two) different schedules the higher daily dose produced about twice the hair cell loss; therefore, the ototoxic effect depends on the total dose but even more on the increased blood concentration.