In the present study, we have investigated the effect of intraperitoneal injection of ethanol (3.5 g/kg) on tyrosine phosphorylation in rat brain. Immunoblot analysis using an antiphosphotyrosine antibody revealed that a 130‐kDa protein band was detected in the brain extract in response to ethanol administration. This ethanol‐stimulated tyrosine phosphorylation of the 130‐kDa protein was found in the brain but not in the heart, liver or thymus. The 130‐kDa phosphotyrosine‐containing protein was identified by immunoprecipitation to be Cas, a crk‐associated src substrate. This ethanol‐stimulated tyrosine phosphorylation of Cas was observed most prominently in the cerebellum and the cerebral cortex. We further examined the possible involvement of Fyn kinase in ethanol‐stimulated Cas tyrosine phosphorylation. Immunecomplex kinase assay showed that Fyn was activated in the cerebellum and cerebral cortex of ethanol‐administered rats. Immunoprecipitation experiments also showed that Fyn was co‐immunoprecipitated with an anti‐Cas antibody in these regions from ethanol‐administered rats. Furthermore, exogenous Fyn was shown to phosphorylate Cas from cerebellum and cerebral cortex in vitro. These findings indicate that ethanol stimulates tyrosine phosphorylation of Cas in rat cerebellum and cerebral cortex, and that Fyn may be involved in the process.