Tobramycin 5.1 mg/kg/day was administered for 9 days to 20 healthy human volunteers to determine the potential vestibulotoxicity of once daily versus thrice daily treatment regimens. Vestibular function was tested carefully before, during, and following drug administration, using a battery of postural and caloric tests. Although subjects receiving the once daily regimen performed better on the postural tests, we feel this does not necessarily indicate an adverse effect in either group. The caloric test was the primary measure of vestibular function in this experiment, and this test appeared to demonstrate a slight adverse effect during drug administration of equal magnitude in both treatment groups. The results of this experiment indicate that there is no significant difference in the risk of adverse vestibular effects between once daily and thrice daily tobramycin administration.