Creatine has in recent years raised the interest of the neurologist, because it has been used in children with hereditary disorders of creatine metabolism and because experimental data suggest that it may exert a protective effect against various neurological diseases including stroke. Moreover, it is widely used as a nutritional supplement. It is well known that creatine crosses the blood-brain barrier with difficulty, however its accumulation into the brain after systemic administration is still not completely known. In the present experiments we studied its accumulation into rat brain tissue after intraperitoneal (i.p.) single or repeated injections. After a single injection of 160 mg/kg, radioactively labelled creatine (14C-creatine) entered the brain to a limited extent. It reached a plateau value of around 70 microM above baseline, that remained stable for at least 9 h. This amount of exogenous creatine obviously added to the endogenous creatine store. This increase is a minor one, since endogenous creatine has a brain concentration of about 10 mM. In accordance with this conclusion, when single or repeated injections of unlabelled ('cold') creatine were administered to rats, no sizable increase could be measured with high-performance liquid chromatography in the brain levels of either this compound or its phosphorylated derivative, phosphocreatine. Although our data clearly show some passage of serum creatine into the brain, other strategies are needed to improve passage of creatine across the blood-brain barrier in a way that it may be suitable to treat acute conditions like stroke.