We describe a method for determination of guanidino compounds in plasma and erythrocytes. The compounds are separated by liquid chromatography, the concentrations in the effluent being estimated fluorometrically (Trans. Am. Soc. Artif. Intern. Organs 24: 61, 1978). Guanidinoacetic acid, guanidinosuccinic acid, guanidinobutyric acid, guanidine, methylguanidine, taurocyamine, and arginine can be detected and quantitated in both plasma and erythrocytes from healthy individuals. The method was also applied to patients in chronic renal failure. Guanidinosuccinic acid, creatinine, guanidine, and methylguanidine are substantially increased in the plasma and erythrocytes of the uremic patient. Guanidino compounds can be effectively extracted from plasma with 0.5 volume of 300 g/L trichloroacetic acid, from erythrocytes with five volumes of 120 g/L trichloroacetic acid.