The pharmacokinetics of gatifloxacin (400 mg orally) and the influence of the antacid aluminum magnesium hydroxide (20 ml of Maalox 70) on the bioavailability of gatifloxacin in 24 healthy volunteers were assessed. In an open, randomized, six-period crossover study, the volunteers received either gatifloxacin alone (treatments A and D); aluminum magnesium hydroxide concomitant with gatifloxacin (treatment C); or aluminum magnesium hydroxide 2 h before (treatment B), 2 h after (treatment E), or 4 h after gatifloxacin administration (treatment F). Gatifloxacin concentrations were measured by a validated bioassay and high-performance liquid chromatography. Pharmacokinetics of a single 400-mg dose of gatifloxacin alone were characterized as follows (mean ± standard deviation): peak concentration (C max), 3.8 ± 0.5 (treatment A) and 3.4 ± 0.9 (treatment D) μg/ml; time toC max, 1.4 ± 0.8 (treatment A) and 1.7 ± 0.7 (treatment D) h; area under the curve from time zero to infinity (AUC0–∞), 33.5 ± 5.9 (treatment A) and 31.4 ± 3.4 (treatment D) μg · h/ml; urine recovery, (83 ± 6)% (treatment A) and (84 ± 8)% (treatment D). Comparison of the results obtained by bioassay showed a good correlation. Aluminum magnesium hydroxide administration 2 h before (treatment B) or concomitant with (treatment C) gatifloxacin decreased the C max by 45% (2.1 ± 1.2 μg/ml) or even 68% (1.2 ± 0.4 μg/ml) highly significantly (P < 0.01). AUC0–∞ was significantly reduced from 33.5 ± 5.9 to 19.4 ± 6.9 μg · h/ml (by 42%) or even to 11.9 ± 3.3 μg · h/ml (by 64%) (P < 0.01). If aluminum magnesium hydroxide was given 2 h after gatifloxacin (treatment E), there was no significant reduction of concentration in serum but AUC0–∞ was significantly reduced from 31.4 ± 3.4 to 25.9 ± 5.3 μg · h/ml (18%) (P < 0.01). Aluminum magnesium hydroxide given 4 h after gatifloxacin (treatment F) showed no influence on the gatifloxacin pharmacokinetics. Therefore, the optimal time between gatifloxacin application and the intake of an aluminum-containing antacid should be 4 h.