The binding of basic compounds to bovine serum albumin (BSA) was studied by the fluorescence method, using N-phenyl-1-naphthylamine (NPN) and 1-anilinonaphthalene-8-sulfonate (ANS) as fluorescent probes. The spectral property of these probes was used for an indirect measurement of the binding of basic compounds to BSA. The compounds used in the present investigation were chlorpromazine, diazepam, chlordiazepoxide, nitrazepam, aniline, and acetanilide. Its results demonstrated that these compounds bind to the hydrophobic regions on the albumin molecule. It appeared that these compounds may be located at least near a tryptophan residue of albumin and two probes may bind at the same site. In the case of chlorpromazine, this drug competed with NPN but not with ANS in binding to BSA. Competitive binding between these compounds, except chlorpromazine, and the probes was observed. The significance of these results was discussed in relation to various types of binding forces. NPN is a useful probe in the evaluation of interactions of basic compounds to serum albumin, but the usefulness of the probe technique may be limited in that binding studies are related only to the site to which the probe binds.