Twelve guanidino compounds were determined in simultaneously sampled serum and cerebrospinal fluid of eight non-dialyzed patients with renal insufficiency. Liquid cation exchange chromatography with a highly sensitive fluorescence detection method was used. In patients with serum urea levels about 10 times higher than in controls, the levels of guanidinosuccinic acid, creatinine, guanidine and methylguanidine, in serum as well as in cerebrospinal fluid, are at least 10 times higher than in control subjects. The levels of argininic acid and N-alpha-acetylarginine (in serum) and gamma-guanidinobutyric acid (in cerebrospinal fluid) are slightly increased (less than 10 X). The levels of the other guanidino compounds are close to normal values. A significant positive correlation exists between the guanidinosuccinic acid, creatinine and guanidine levels in serum and cerebrospinal fluid. The accumulation of several experimentally proven toxic guanidino compounds could contribute to the complex nervous system symptomatology and the hematological complications seen in renal insufficiency.