The displacement of protein bound sulfonamides (sulfisoxazole, sulfamethoxazole, sulfisomidine) by furosemide was investigated in bovine serum albumin by equilibrium dialysis method. Furosemide $(2{ imes}10^{-4}M)$ in bovine serum albumin ($7.24{ imes}10^{-5}$, $1.45{ imes}10^{-4}$, $2.89{ imes}10^{-4}M$). Sulfisoxa캐1e and furosemide were bound reversibly to bovine serum albumin and competitive for the same binding sites when administered together. Consequently, dosage regimen of sulfisoxazole should be adjusted carefully when sulfisoxazole is administered along with furosemide.