The interaction between drugs (indomethacin (ID), p-chlorobenzamide (CBA) and 5-methoxy-2-methyl-3-indoleacetic acid (MMI)) and bovine serum albumin (BSA) was investigated by equilibrium dialysis and nuclear magnetic resonance (n.m.r.) spectroscopy. The binding of ID and MMI to BSA was concluded to be hydrophobic and hydrophilic, respectively, on the basis of the dependence of the binding constants on temperature and ionic strength. In H-1 n.m.r. spectra of ID, there were no significant shifts with change in the concentration and on addition of BSA. The spin-lattice relaxation time (T1) and spin-spin relaxation time (T2) of the respective protons of ID were independent of concentration, but depended on the concentration of BSA added. The binding position was determined from the ratio of the spin-spin relaxation rates of ID bound to BSA and free ID. ID and MMI were found to bind to BSA through the aromatic moiety and the carboxyl group as the substituent, respectively. The binding property of ID was known to be governed by the competition between the hydrophobic effect of the chlorophenyl group and the hydrophilic effect of the carboxyl group in the molecule.