Abstract—Mitochondria isolated from rat brain contained ribosomes, with an apparent sedimentation constant (j) of 60, which were dissociable into 45sand 32ssubunits. The RNA associated with the 45ssubunit had ansvalue of 16; the RNA of the 32ssubunit had ansvalue of 13. These values were essentially like those found for the mitochondria of rat liver. Nascent protein associated with the 60smonosome was reduced by chloramphenicol but not by cycloheximide. All cycloheximide‐sensitive activity found in brain mitochondria prepared from sucrose or Ficoll‐sucrose gradients and most of that associated with nerve‐ending preparations that were similarly prepared could be attributed to a contaminant that was probably a membrane‐enclosed, cytoplasmic‐like system which contained, in addition to 18sand 28sRNA, one or more mitochondria. Nerve‐ending preparations from which most or all of this contamination had been removed exhibited very little cycloheximide‐sensitive protein syn