Phase I and phase II biochemical reactions involved in the biotransformation pathways of tolvaptan were characterized by LC-MS-based techniques and in vitro models to identify the most appropriate marker(s) of intake. The effects of physiological and non-physiological factors on the metabolic profile of tolvaptan were also evaluated. In vitro approaches were based on the use of pooled human liver microsomes and recombinant isoforms of cytochrome P450 and uridine diphospho glucuronosyl-transferase. Sample preparation included liquid/liquid extraction at neutral pH with tert-butyl methyl-ether. In the case of the study of phase II metabolism an additional enzymatic hydrolysis step was performed. The chromatographic separation was carried out using reversed-phase chromatography, whereas detection was performed by either triple-quadrupole or time-of-flight analyzers in positive electrospray ionization and different acquisition modes. Our data show that tolvaptan is metabolized to at least 20 phase I metabolites, the biotransformation reactions being catalyzed mainly by CYP3A4 and CYP3A5 isoforms. The phase-I reactions include hydroxylation (in different positions), carboxylation, oxidation, hydrogenation, dealkylation, isomerization and a combination of the above. Most of the phase I metabolites undergo glucuronidation, carried out mostly by UGT2B7 and UGT2B17 isoforms. Dealkylated, mono-hydroxylated and carboxylated metabolites both in the free and in the glucuronidated form appear to be the most suitable urinary diagnostic markers for the detection of tolvaptan intake in doping control. Concerning the effects of physiological and non-physiological factors on the metabolic profile of tolvaptan, our results show that (i) no significant gender differences were detected; (ii) significant differences were registered in the presence of different CYP3A5 allelic variants, and finally (iii) a marked reduction of the detected metabolites was registered in the presence of antifungals, and, to a lesser extent, of steroidal progestins.