A pharmacokinetic comparison of the two recommended dosages of vancomycin given as multiple doses has not been previously performed. Eleven adult subjects with normal renal function randomly received 500 mg every 6 h (five doses) and, later, 1,000 mg every 12 h (three doses). Each dose was infused over 1 h, and regimens were separated by 1 week. Compared with the two-compartment fit, a three-compartment fit significantly reduced the residual weighted sums of squares. Accumulation occurred for both regimens after repeated dosing and was independent of dose. At steady state, concentrations in serum at 1 h showed little variation for the 1,000- or the 500-mg dose regimen (33.7 +/- 3.8 versus 22.6 +/- 3.2 micrograms/ml); trough concentrations were 7.9 +/- 1.7 versus 11.2 +/- 2.2 micrograms/ml, respectively. With the 1,000-mg dose, the terminal half-life was 7.7 +/- 1.8 h, steady-state area under the curve for the dose interval was 227 +/- 28.3 micrograms X h/ml, and total body clearance was 86.1 +/- 8.9 ml/min per 1.73 m2. The red-man syndrome occurred in 9 of 11 volunteers who received 1,000-mg doses and in none of those who received 500-mg doses. We concluded that vancomycin disposition in healthy adults with normal renal function is best described by a three-compartment model, there is relatively little variation in vancomycin disposition in normal volunteers, significant accumulation occurs with multiple dosing, it is inappropriate to use the same therapeutic window for both regimens, and the pharmacokinetics of vancomycin justify a 12-h dose interval; however, a 1-g dose is associated with a significantly greater incidence of the red-man syndrome.