When extracts of brain from normal and dystrophic chickens were incubated at 30oC under conditions that favored microtubule assembly, the increase in turbidity of the extract from the dystrophic animal was approximately 50% less than the increase in the turbidity of the extract from the normal animal. In developmental studies using age-matched normal and dystrophic chickens this difference in turbidity was observed only with chickens 65 days ex-ovo or older. The extent of microtubule assembly, as determined by a sedimentation methodology, was a linear function of the total protein concentration in the extract and was equal in extracts from normal and dystrophic chickens. In contrast, the total increase in turbidity was much larger than could be accounted for by sedimenting microtubules and varied in a non-linear manner with the total protein concentration. There was no increase in turbidity when brain extracts that were prepared by centrifugation at 100,000gx 40 min were incubated at 30oC. The condensation reaction of the tubulin in these high speed extracts with microtubules was equal in the extracts from normal and dystrophic animals. These studies indicate that a developmentally regulated difference in brain extracts from normal and dystrophic chickens can be demonstrated by turbidimetric methods but this difference is unrelated to the formation of microtubules in these extracts. These studies support the concept that muscular dystrophy is also accompanied by a change in the central nervous system.