Previous observations have demonstrated decreased in vivo and in vitro protein synthesis by brain ribosomal systems following long-term ethanol ingestion. For further investigation of the properties of brain ribosomes, the 40s and 60s ribosomal subunits were successfully isolated from control and chronic 10% ethanol-drinking rats. For a successful dissociation of ribosomes into subunits NH4C1, puromycin and a high-salt treatment at 10Β°C were essential with a critical concentration of Mg2+ since ribosomes could not be resolved at less than 7 mM Mg2+. Analysis of the A23, profile of the subunits o n the sucrose gradients showed no significant differences between the control and ethanol-ingesting groups. Studies on H-labeled ribosomes following in vivo RNA labeling showed correspondence of the radioactive profiles from the incorporation of [ S 3 H ] orotic acid into RNA with the sucrose gradient absorbance profile of 6 0 s and 40s ribosomal subunits. Furthermore, active reassociation of both subunits occurred at 37OC as demonstrated by the increased [ l4c1phenylalanine incorporation in the presence of poly(U). Results further showed that the poly(U)-dependent [ 14C] phenylalanine incorporation was significantly reduced by the subunits from the ethanol-ingesting animals. These findings suggest that long-term ingestion of ethanol caused functional changes in the properties of brain ribosomes, specifically on the reassociation process of the two subunits.