When co-administered with ketoconazole, mean C(max) and AUC(0,∞) of tolvaptan were increased 3.48- and 5.40-fold, respectively. Twenty-four hour urine volume increased from 5.9 to 7.7 l. Erythromycin breath testing showed no difference following a single dose of tolvaptan. With rifampicin, tolvaptan mean C(max) and AUC were reduced to 0.13- and 0.17-fold of tolvaptan administered alone. Twenty-four hour urine volume decreased from 12.3 to 8.8 l.