Guanidino compounds have been suggested to contribute to the complex neurological complications associated with uremia. Several of them have previously been reported to accumulate in physiological fluids of renal insufficient subjects. We report on guanidino compound levels in 28 brain regions in control and uremic brains. In all brain regions studied, in controls as well as in uremic patients, concentrations of alpha-keto-delta-guanidinovaleric acid, alpha-N-acetylarginine and beta-guanidinopropionic acid remained below detection limits. Creatine, guanidinoacetic acid, argininic acid, gamma-guanidinobutyric acid, arginine and homoarginine were not increased in uremic patients. Argininic acid and homoarginine were detectable in some brain regions only. Creatine concentrations varied from 2500 +/- 2100 nmol/g tissue in hypophysis to 10500 +/- 1200 nmol/g tissue in cerebellar cortex. Even more pronounced regional differences were found for gamma-guanidinobutyric acid with the lowest concentration in the caudate nucleus (0.6 +/- 0.3 nmol/g tissue) and highest in substantia nigra, pallidum and cerebellar dentate nucleus (8.3 +/- 2.8 nmol/g tissue). The guanidinosuccinic acid levels were below detection limit in controls in the majority of brain regions. Taking into account the detection limit of guanidinosuccinic acid for a certain amount of tissue applied to the analytical system, important increases (approx. up to > 100 fold) were observed in all brain regions of uremic patients. Accumulation of guanidinosuccinic acid increased with increasing degree of renal failure with levels up to 65 nmol/g tissue in the hypophysis. Creatinine concentrations were also found to be increased in uremic brain regions but increases seemed to be less strictly related to serum urea levels. Guanidine and methylguanidine were found only occasionally in brain regions of controls while respectively 100- and 30-fold increases were found in brain regions of uremic subjects. Levels of guanidinosuccinic acid and creatinine in uremic brain were comparable to those previously observed in brain of experimental animals displaying convulsions following intraperitoneal injection of the respective compounds. Our findings further establish guanidino compounds as probable uremic toxins contributing to the neurological complications in uremia.