Synaptosomes and lysosomes of rat brain were separated by differential centrifugation and a two-step gradient centrifugation with colloidal silica-gel (Percoll). The organelles were identified by the measurement of established marker-enzymes and by electronmicroscopy. Renin activity, measured by radioimmunoassay for angiotensin I (ANG I), was localized in the synaptosomes and cathepsin D-activity was found in the lysosomal fraction. Converting-enzyme activity was present in the renin-containing synaptosomes. Part of the brain renin activity could be activated by pre-incubation with trypsin. Affinity chromatography of an organelle-enriched brain fraction was carried out using a caseinyl-sepharose column and resulted in the separation of renin from cathepsin D activity; the renin peak was inhibited by antibodies raised against rat kidney renin. We conclude, that the formation of ANG I and its activation to angiotensin II (ANG II) by converting enzyme is possible in synaptosomes. This adds further evidence to an intraneuronal synthesis of ANG I and ANG II in the brain and is in support of previous results demonstrating an intraneuronal localization of the components of the brain renin-angiotensin system.