The cerebral distribution of ³H-atropine sulphate and ³H-methylatropine nitrate in rats was investigated 30 min after intraperitoneal administration of both drugs. Both compounds were found to be pure (>98%) by paper chromatography and paper electrophoresis. The distribution of radioactivity over various metabolites in liver, plasma and CNS was also carried out by paper chromatography and paper electrophoresis. Radioactivity in the CNS after ³H-atropine administration is about three times higher than it is after injection of ³H-methylatropine. The penetration of radioactivity into the CNS, however, is very low. It is estimated that following the systemic administration of a dose of 10 mg/kg, 0.100 nmol atropine or 0.012 nmol methylatropine per 100 mg fresh brain tissue is present after 30 min. The distribution of atropine over eleven parts of the brain tends to be homogenous, although there are indications that atropine is taken up preferentially in cerebellum, pons and hypothalamus. Methylatropine is preferentially taken up by cerebellum, pons, preoptic and septal area. The significance of these findings for studies in which methylatropine is applied locally in the brain, is discussed. It is concluded that blockade of cholinergic receptors in the CNS is possible after peripheral administration of atropine and methylatropine as well.