Comparative study of haloperidol (HPD), biperiden (BPD) and clonazepam (CNZ) interactions with human and bovine serum albumin was performed based on fluorescence quenching analysis. We used mathe matical modeling comparing spectrofluorimetric data to obtain information on the possibility of compe tition among three drugs by sites binding. Results showed that the three drugs studied have high affinity for albumin and suggest the existence of two site classes in HSA for HPD and only one class for BPD and CNZ, in the range of concentrations tested for each drug. Among them, only HPD forms complex with HSA. Comparing normalized quenching plots suggested that the primary sites in HSA and BSA for HPD and CNZ are located at subdomain IB, whereas BPD would bind in the subdomain IIA. Considering the competition for binding sites in HSA, titrations of HPD-HSA complex by BPD and CNZ, as well as the titra tion of HSA solution containing CNZ titrated by BPD, show that although the three drugs do not compete with each other for binding sites, their interaction with HSA can cause conformational change in the pro tein, and to increase or decrease the accessibility to binding sites for other drug. This may mean alteration in the free plasma drug concentrations.